diff --git a/src/Activity.php b/src/Activity.php
deleted file mode 100644
index dae7d1b..0000000
--- a/src/Activity.php
+++ /dev/null
@@ -1,30 +0,0 @@
-<?php
-
-namespace Zoomyboy\LaravelNami;
-
-use Illuminate\Database\Eloquent\Model;
-
-class Activity extends Model
-{
-    protected $guarded = [];
-
-    public static function fromNami($item)
-    {
-        $item = collect($item)
-            ->only(['descriptor', 'id'])
-            ->mapWithKeys(function ($item, $key) {
-                if ('id' == $key) {
-                    return ['id' => $item];
-                }
-
-                return ['name' => $item];
-            })->toArray();
-
-        return new self($item);
-    }
-
-    public function subactivities()
-    {
-        return Nami::subactivitiesOf($this->id);
-    }
-}
diff --git a/src/Api.php b/src/Api.php
index b3c7b55..2f07d83 100644
--- a/src/Api.php
+++ b/src/Api.php
@@ -10,6 +10,9 @@ use Illuminate\Support\Collection;
 use Illuminate\Support\LazyCollection;
 use Illuminate\Support\Str;
 use Zoomyboy\LaravelNami\Authentication\Authenticator;
+use Zoomyboy\LaravelNami\Data\Activity;
+use Zoomyboy\LaravelNami\Data\Group;
+use Zoomyboy\LaravelNami\Data\Subactivity;
 use Zoomyboy\LaravelNami\Data\Baustein;
 use Zoomyboy\LaravelNami\Data\Course;
 use Zoomyboy\LaravelNami\Data\Member;
@@ -198,14 +201,17 @@ class Api
         }
     }
 
-    public function subactivitiesOf(int $activityId): Collection
+    /**
+     * @return Collection<int, Subactivity>
+     */
+    public function subactivitiesOf(Activity $activity): Collection
     {
         $this->assertLoggedIn();
 
         return $this->fetchCollection(
-            '/ica/rest/nami/untergliederungauftaetigkeit/filtered/untergliederung/taetigkeit/'.$activityId,
+            '/ica/rest/nami/untergliederungauftaetigkeit/filtered/untergliederung/taetigkeit/'.$activity->id,
             'Fetch subactivities failed'
-        )->map(fn ($subactivity) => Subactivity::fromNami($subactivity));
+        )->map(fn ($subactivity) => Subactivity::from($subactivity));
     }
 
     public function membership(int $memberId, int $membershipId): Membership
@@ -318,14 +324,17 @@ class Api
         return false !== $this->groups()->search(fn ($group) => $group->id == $groupId);
     }
 
-    public function groups(int $parentGroupId = null): Collection
+    /**
+     * @return Collection<int, Group>
+     */
+    public function groups(?Group $parentGroup = null): Collection
     {
         $this->assertLoggedIn();
 
         return $this->fetchCollection(
-            '/ica/rest/nami/gruppierungen/filtered-for-navigation/gruppierung/node/'.($parentGroupId ?: 'root'),
+            '/ica/rest/nami/gruppierungen/filtered-for-navigation/gruppierung/node/'.($parentGroup ? $parentGroup->id : 'root'),
             'Group fetch failed'
-        )->map(fn ($group) => Group::fromResponse($group, $parentGroupId));
+        )->map(fn ($group) => Group::from([...$group, 'parentId' => $parentGroup ? $parentGroup->id : null]));
     }
 
     public function group(int $groupId): ?Group
@@ -335,13 +344,6 @@ class Api
         return $this->groups()->first(fn ($group) => $group->id == $groupId);
     }
 
-    public function subgroupsOf(int $groupId): Collection
-    {
-        $this->assertLoggedIn();
-
-        return $this->groups($groupId);
-    }
-
     public function genders(): Collection
     {
         $this->assertLoggedIn();
@@ -392,12 +394,15 @@ class Api
             ->map(fn ($confession) => Confession::fromNami($confession));
     }
 
-    public function activities(int $groupId): Collection
+    /**
+     * @return Collection<int, Activity>
+     */
+    public function activities(Group $group): Collection
     {
         $this->assertLoggedIn();
 
-        return $this->fetchCollection("/ica/rest/nami/taetigkeitaufgruppierung/filtered/gruppierung/gruppierung/{$groupId}", 'Fetch activities failed')
-            ->map(fn ($activity) => Activity::fromNami($activity));
+        return $this->fetchCollection("/ica/rest/nami/taetigkeitaufgruppierung/filtered/gruppierung/gruppierung/{$group->id}", 'Fetch activities failed')
+            ->map(fn ($activity) => Activity::from($activity));
     }
 
     /**
diff --git a/src/Casters/GroupCast.php b/src/Casters/GroupCast.php
new file mode 100644
index 0000000..883c1be
--- /dev/null
+++ b/src/Casters/GroupCast.php
@@ -0,0 +1,25 @@
+<?php
+
+namespace Zoomyboy\LaravelNami\Casters;
+
+use Spatie\LaravelData\Casts\Cast;
+use Spatie\LaravelData\Support\DataProperty;
+
+class GroupCast implements Cast
+{
+    public function __construct()
+    {
+    }
+
+    /**
+     * @param array<string, mixed> $context
+     */
+    public function cast(DataProperty $property, mixed $value, array $context): ?string
+    {
+        if ($property->type->isNullable && !$value) {
+            return null;
+        }
+
+        return $value;
+    }
+}
diff --git a/src/Data/Activity.php b/src/Data/Activity.php
new file mode 100644
index 0000000..88cbd7f
--- /dev/null
+++ b/src/Data/Activity.php
@@ -0,0 +1,17 @@
+<?php
+
+namespace Zoomyboy\LaravelNami\Data;
+
+use Illuminate\Support\Collection;
+use Zoomyboy\LaravelNami\Nami;
+
+class Activity extends EnumData
+{
+    /**
+     * @return Collection<int, Subactivity>
+     */
+    public function subactivities(): Collection
+    {
+        return Nami::subactivitiesOf($this);
+    }
+}
diff --git a/src/Data/Group.php b/src/Data/Group.php
new file mode 100644
index 0000000..2d272b2
--- /dev/null
+++ b/src/Data/Group.php
@@ -0,0 +1,33 @@
+<?php
+
+namespace Zoomyboy\LaravelNami\Data;
+
+use Illuminate\Support\Collection;
+use Spatie\LaravelData\Attributes\MapInputName;
+use Zoomyboy\LaravelNami\Nami;
+
+class Group extends EnumData
+{
+    public function __construct(
+        public int $id,
+        #[MapInputName('descriptor')]
+        public string $name,
+        public ?int $parentId,
+    ) {}
+
+
+    /**
+     * @return Collection<int, Activity>
+     */
+    public function activities(): Collection
+    {
+        return Nami::activities($this);
+    }
+
+    /**
+     * @return Collection<int, self>
+     */
+    public function children(): Collection {
+        return Nami::groups($this);
+    }
+}
diff --git a/src/Data/Membership.php b/src/Data/Membership.php
index e656421..1f998d3 100644
--- a/src/Data/Membership.php
+++ b/src/Data/Membership.php
@@ -49,4 +49,9 @@ class Membership extends Data
             'untergliederungId' => $this->subactivityId,
         ];
     }
+
+    public function group(): Group {
+        return Group::from(['name' => $this->group ?: '', 'id' => $this->groupId]);
+    }
+
 }
diff --git a/src/Data/Subactivity.php b/src/Data/Subactivity.php
new file mode 100644
index 0000000..37a8648
--- /dev/null
+++ b/src/Data/Subactivity.php
@@ -0,0 +1,8 @@
+<?php
+
+namespace Zoomyboy\LaravelNami\Data;
+
+class Subactivity extends EnumData
+{
+
+}
diff --git a/src/Group.php b/src/Group.php
deleted file mode 100644
index 67f220d..0000000
--- a/src/Group.php
+++ /dev/null
@@ -1,62 +0,0 @@
-<?php
-
-namespace Zoomyboy\LaravelNami;
-
-use Illuminate\Contracts\Support\Arrayable;
-use Illuminate\Support\Collection;
-
-class Group implements Arrayable
-{
-    public string $name;
-    public int $id;
-    public ?int $parentId;
-
-    public static function fromResponse(array $response, ?int $parent): self
-    {
-        return (new self())
-            ->setName($response['descriptor'])
-            ->setId($response['id'])
-            ->setParentId($parent);
-    }
-
-    public function setName(string $name): self
-    {
-        $this->name = $name;
-
-        return $this;
-    }
-
-    public function setId(int $id): self
-    {
-        $this->id = $id;
-
-        return $this;
-    }
-
-    public function setParentId(?int $parentId = null): self
-    {
-        $this->parentId = $parentId;
-
-        return $this;
-    }
-
-    public function toArray()
-    {
-        return ['id' => $this->id, 'name' => $this->name, 'parent_id' => $this->parentId];
-    }
-
-    public function subgroups()
-    {
-        return Nami::subgroupsOf($this->id);
-    }
-
-    public function fees()
-    {
-        return Nami::feesOf($this->id);
-    }
-
-    public function activities(): Collection
-    {
-        return Nami::activities($this->id);
-    }
-}
diff --git a/src/Subactivity.php b/src/Subactivity.php
deleted file mode 100644
index 9c2cd3f..0000000
--- a/src/Subactivity.php
+++ /dev/null
@@ -1,35 +0,0 @@
-<?php
-
-namespace Zoomyboy\LaravelNami;
-
-use Illuminate\Database\Eloquent\Model;
-
-class Subactivity extends Model
-{
-    protected $guarded = [];
-
-    public static function fromNami($item)
-    {
-        $item = collect($item)
-            ->only(['descriptor', 'id'])
-            ->mapWithKeys(function ($item, $key) {
-                if ('id' == $key) {
-                    return ['id' => $item];
-                }
-
-                return ['name' => $item];
-            })->toArray();
-
-        return new self($item);
-    }
-
-    public function getNameAttribute()
-    {
-        return ucfirst($this->attributes['name']);
-    }
-
-    public function getIsNullAttribute()
-    {
-        return $this->attributes['id'] == self::getNullValue();
-    }
-}
diff --git a/tests/Unit/Api/MembershipTest.php b/tests/Unit/Api/MembershipTest.php
index 6e95c53..c02ff02 100644
--- a/tests/Unit/Api/MembershipTest.php
+++ b/tests/Unit/Api/MembershipTest.php
@@ -35,7 +35,8 @@ class MembershipTest extends TestCase
         $this->assertSame(1000, $membership->groupId);
         $this->assertSame(15, $membership->activityId);
         $this->assertSame(16, $membership->subactivityId);
-        $this->assertSame('::group::', $membership->group);
+        $this->assertSame('::group::', $membership->group()->name);
+        $this->assertSame(1000, $membership->group()->id);
     }
 
     public function testFetchesMembership(): void
diff --git a/tests/Unit/GetGroupsTest.php b/tests/Unit/GetGroupsTest.php
index f579886..e9e93af 100644
--- a/tests/Unit/GetGroupsTest.php
+++ b/tests/Unit/GetGroupsTest.php
@@ -3,7 +3,7 @@
 namespace Zoomyboy\LaravelNami\Tests\Unit;
 
 use Illuminate\Support\Facades\Http;
-use Zoomyboy\LaravelNami\Group;
+use Zoomyboy\LaravelNami\Data\Group;
 use Zoomyboy\LaravelNami\Tests\TestCase;
 
 class GetGroupsTest extends TestCase
@@ -18,11 +18,11 @@ class GetGroupsTest extends TestCase
             'https://nami.dpsg.de/ica/rest/nami/gruppierungen/filtered-for-navigation/gruppierung/node/root' => Http::response($this->groupsResponse, 200),
         ]);
 
-        $groups = $this->login()->groups();
+        $group = $this->login()->groups()->first();
 
-        $this->assertEquals([
-            ['id' => 100, 'name' => 'Group', 'parent_id' => null],
-        ], $groups->toArray());
+        $this->assertSame(100, $group->id);
+        $this->assertSame('Group', $group->name);
+        $this->assertNull($group->parentId);
         Http::assertSent(function ($request) {
             return 'https://nami.dpsg.de/ica/rest/nami/gruppierungen/filtered-for-navigation/gruppierung/node/root' == $request->url();
         });
@@ -49,13 +49,13 @@ class GetGroupsTest extends TestCase
             'https://nami.dpsg.de/ica/rest/nami/gruppierungen/filtered-for-navigation/gruppierung/node/100' => Http::response($this->subgroupsResponse, 200),
         ]);
 
-        $subgroups = $this->login()->group(100)->subgroups();
+        $subgroups = $this->login()->group(100)->children();
 
         $this->assertEquals([
-            ['id' => 101300, 'parent_id' => 100, 'name' => 'Siebengebirge'],
-            ['id' => 100900, 'parent_id' => 100, 'name' => 'Sieg'],
-            ['id' => 100900, 'parent_id' => 100, 'name' => 'Sieg'],
-            ['id' => 101000, 'parent_id' => 100, 'name' => 'Voreifel'],
+            ['id' => 101300, 'parentId' => 100, 'name' => 'Siebengebirge'],
+            ['id' => 100900, 'parentId' => 100, 'name' => 'Sieg'],
+            ['id' => 100900, 'parentId' => 100, 'name' => 'Sieg'],
+            ['id' => 101000, 'parentId' => 100, 'name' => 'Voreifel'],
         ], $subgroups->toArray());
         $subgroups->each(function ($group) {
             $this->assertInstanceOf(Group::class, $group);
diff --git a/tests/Unit/GroupsTest.php b/tests/Unit/GroupsTest.php
index 53b6eca..c153b7d 100644
--- a/tests/Unit/GroupsTest.php
+++ b/tests/Unit/GroupsTest.php
@@ -7,7 +7,7 @@ use Zoomyboy\LaravelNami\Exceptions\NoJsonReceivedException;
 use Zoomyboy\LaravelNami\Exceptions\NotAuthenticatedException;
 use Zoomyboy\LaravelNami\Exceptions\NotSuccessfulException;
 use Zoomyboy\LaravelNami\Fakes\GroupFake;
-use Zoomyboy\LaravelNami\Group;
+use Zoomyboy\LaravelNami\Data\Group;
 use Zoomyboy\LaravelNami\Nami;
 use Zoomyboy\LaravelNami\Tests\TestCase;
 
@@ -32,6 +32,7 @@ class GroupsTest extends TestCase
         $this->assertInstanceOf(Group::class, $group);
         $this->assertEquals('testgroup', $group->name);
         $this->assertEquals(1234, $group->id);
+        $this->assertNull($group->parentId);
 
         app(GroupFake::class)->assertRootFetched();
     }
@@ -45,10 +46,11 @@ class GroupsTest extends TestCase
             555 => ['name' => 'ABC'],
         ]);
 
-        $group = Nami::login(12345, 'secret')->groups(1234)->first();
+        $group = Nami::login(12345, 'secret')->groups(Group::from(['id' => 1234, 'name' => 'lorem', 'parentId' => null]))->first();
 
         $this->assertEquals('ABC', $group->name);
         $this->assertEquals(555, $group->id);
+        $this->assertEquals(1234, $group->parentId);
 
         app(GroupFake::class)->assertFetched(1234);
     }
@@ -77,7 +79,7 @@ class GroupsTest extends TestCase
         ]);
         app(GroupFake::class)->failsToFetch(1234);
 
-        Nami::login(12345, 'secret')->groups(1234);
+        Nami::login(12345, 'secret')->groups(Group::from(['id' => 1234, 'name' => 'lorem', 'parentId' => null]));
     }
 
     public function testItDoesntReturnGroupWhenNoJsonIsReturned(): void
diff --git a/tests/Unit/PullActivitiesTest.php b/tests/Unit/PullActivitiesTest.php
index d7d03f9..76c1146 100644
--- a/tests/Unit/PullActivitiesTest.php
+++ b/tests/Unit/PullActivitiesTest.php
@@ -3,6 +3,7 @@
 namespace Zoomyboy\LaravelNami\Tests\Unit;
 
 use Illuminate\Support\Facades\Http;
+use Zoomyboy\LaravelNami\Data\Activity;
 use Zoomyboy\LaravelNami\Exceptions\NoJsonReceivedException;
 use Zoomyboy\LaravelNami\Exceptions\NotSuccessfulException;
 use Zoomyboy\LaravelNami\Fakes\SubactivityFake;
@@ -22,8 +23,8 @@ class PullActivitiesTest extends TestCase
         $activities = $this->login()->group(103)->activities();
 
         $this->assertSame([
-            ['name' => 'Ac1', 'id' => 4],
-            ['name' => 'Ac2', 'id' => 3],
+            ['id' => 4, 'name' => 'Ac1'],
+            ['id' => 3, 'name' => 'Ac2'],
         ], $activities->toArray());
         Http::assertSentCount(2);
     }
@@ -39,8 +40,8 @@ class PullActivitiesTest extends TestCase
         $subactivities = $this->login()->group(103)->activities()->first()->subactivities();
 
         $this->assertSame([
-            ['name' => 'Biber', 'id' => 40],
-            ['name' => 'Wölfling', 'id' => 30],
+            ['id' => 40, 'name' => 'Biber'],
+            ['id' => 30, 'name' => 'Wölfling'],
         ], $subactivities->toArray());
         Http::assertSentCount(3);
     }
@@ -50,7 +51,7 @@ class PullActivitiesTest extends TestCase
         $this->expectException(NotSuccessfulException::class);
         app(SubactivityFake::class)->fetchFails(4, 'sorry dude');
 
-        $subactivities = $this->login()->subactivitiesOf(4);
+        $this->login()->subactivitiesOf(Activity::from(['id' => 4]));
     }
 
     public function testContinueIfSubactivitiesRequestReturnsHtml(): void
@@ -58,6 +59,6 @@ class PullActivitiesTest extends TestCase
         $this->expectException(NoJsonReceivedException::class);
         app(SubactivityFake::class)->fetchFailsWithoutJson(4);
 
-        $this->login()->subactivitiesOf(4);
+        $this->login()->subactivitiesOf(Activity::from(['id' => 4]));
     }
 }