Extend the Unity Toolbar with your own UI code. Please note that it's quite hacky as the code is relying on reflection to access Unity's internal code. It might not work anymore with a new Unity update but is proven to work up to (at least) Unity 2021.2.

Add buttons to quickly access scenes, add sliders, toggles, anything.

I added buttons for control Time scale from editor panel. Add from examples after importing. Buttons display in play mode only.

To use this in your Unity project import it from Unity Package Manager. You can download it and import it from your hard drive, or link to it from github directly.

This example code is shown in action in the gif below. Just hook up your GUI method to ToolbarExtender.LeftToolbarGUI or ToolbarExtender.RightToolbarGUI to draw left and right from the play buttons.

	[InitializeOnLoad]
	public class SceneSwitchLeftButton
	{
		static SceneSwitchLeftButton()
		{
			ToolbarExtender.LeftToolbarGUI.Add(OnToolbarGUI);
		}

		static void OnToolbarGUI()
		{
			GUILayout.FlexibleSpace();

			if(GUILayout.Button(new GUIContent("1", "Start Scene 1"), ToolbarStyles.commandButtonStyle))
			{
				SceneHelper.StartScene("Assets/ToolbarExtender/Example/Scenes/Scene1.unity");
			}

			if(GUILayout.Button(new GUIContent("2", "Start Scene 2"), ToolbarStyles.commandButtonStyle))
			{
				SceneHelper.StartScene("Assets/ToolbarExtender/Example/Scenes/Scene2.unity");
			}
		}
	}