您可以编写自定义检查器或使用枚举

public enum Shape
{
    Circle,
    Square,
    Triangle
}

并将字段添加到MonoBehaviour中

public Shape shape;

会变成那样

艰难而漫长的路

EditorGUILayout.Popup(indexOfTheSelectedOption, stringArrayOfOptions);

这里有一个例子

[CustomEditor(typeof(MyScript))]
public class MyScriptEditor : Editor
{        
    // the index of selected option
    private int index;

    public override void OnInspectorGUI()
    {
        // the index of selected option
        index = EditorGUILayout.Popup(index, new string[] { "Option1", "etc" });
    }
}

您可以找到更多的here和here。
编辑：
就像你在评论里问的

[CustomEditor(typeof(MyScript))]
public class MyScriptEditor : Editor
{        
    // the serialized fields of the varibles you wanna change
    SerialiedProperty var1, var2, var3; // ...etc.

    // the index of selected option
    private int index;

    private void Awake()
    {
        var1 = serializedObject.FindProperty("var1");
        var2 = serializedObject.FindProperty("var2");
        var3 = serializedObject.FindProperty("var2"); // the string is the name of your field
    }

    public override void OnInspectorGUI()
    {
        // the index of selected option
        index = EditorGUILayout.Popup(index, new string[] { "Option1", "etc" });

        object yourVal;

        switch (index)
        {
        case 0:
            yourVal = EditorGUILayout.FloatField(yourVal);
            break;
        /*
            and etc,
            you create the field in inspector you want for eevery index

            but tbh, that's a bad idea to do so
        */
        }

        serializedObject.ApplyModifiedProperties();
    }
}