Scene picker for Unity3D

Editor\SceneAssetPathFieldPropertyDrawer.cs

using UnityEditor;
using UnityEngine;

[CustomPropertyDrawer(typeof(SceneAssetPathField))]
public class SceneAssetPathFieldPropertyDrawer : PropertyDrawer
{
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        var oldPath = AssetDatabase.LoadAssetAtPath<SceneAsset>(property.stringValue);

        position = EditorGUI.PrefixLabel(position, new GUIContent(property.displayName));

        EditorGUI.BeginChangeCheck();
        var newScene = EditorGUI.ObjectField(position, oldPath, typeof(SceneAsset), false) as SceneAsset;

        if (EditorGUI.EndChangeCheck())
        {
            var newPath = AssetDatabase.GetAssetPath(newScene);
            property.stringValue = newPath;
        }
    }
}

SceneAssetPathFieldAttribute.cs

using UnityEngine;

public class SceneAssetPathField : PropertyAttribute
{
    // While there are various options to tweak the code above
    // to suit other needs (store only name, loadable path, maybe even index - requires editor script rewrite, though)
    // this helper comes in handy for the version found here.
    // Useful because in editor, the scene is saved with Assets/ prefix an *.unity suffix, but
    // in build the prefix is removed and SceneManager works without extension.
    // Not a fully optimized one, but hey - this is just a tip! ;)
    public static string LoadableName(string path)
    {
        string start = "Assets/";
        string end = ".unity";

        if (path.EndsWith(end))
        {
            path = path.Substring(0, path.LastIndexOf(end));
        }

        if (start.StartsWith(start))
        {
            path = path.Substring(start.Length);
        }

        return path;
    }
}

SceneReferencingObject.cs

using UnityEngine;

public class SceneReferencingObject : MonoBehaviour
{
    [SerializeField, SceneAssetPathField]
    private string referencedScenePath;
}