I do enjoy using Unity, but if there is one thing I do not like, it is that when I want to run the project I have open, it always runs the scene I currently have open.

This is not always what I need, in fact, I never really need that. This script based on one I found in the Unity forums (see the comments in the code for original thread) allows me to specify which scene to run.

This is perfect for instances where you want say a menu scene to load rather than the current game scene you are working on.

View the Gist on GitHub here

using UnityEngine;
using UnityEditor;
using UnityEditor.SceneManagement;

/// <summary>
/// Scene auto loader.
/// </summary>
/// <description>
/// This class adds a File > Scene Autoload menu containing options to select
/// a "master scene" enable it to be auto-loaded when the user presses play
/// in the editor. When enabled, the selected scene will be loaded on play,
/// then the original scene will be reloaded on stop.
///
/// Based on an idea on this thread:
/// http://forum.unity3d.com/threads/157502-Executing-first-scene-in-build-settings-when-pressing-play-button-in-editor
/// </description>
[InitializeOnLoad]
static class SceneAutoLoader
{
    // Static constructor binds a playmode-changed callback.
    // [InitializeOnLoad] above makes sure this gets executed.
    static SceneAutoLoader()
    {
        EditorApplication.playModeStateChanged += OnPlayModeChanged;
    }

    // Menu items to select the "master" scene and control whether or not to load it.
    [MenuItem("File/Scene Autoload/Select Master Scene...")]
    private static void SelectMasterScene()
    {
        string masterScene = EditorUtility.OpenFilePanel("Select Master Scene", Application.dataPath, "unity");
        masterScene = masterScene.Replace(Application.dataPath, "Assets");  //project relative instead of absolute path
        if (!string.IsNullOrEmpty(masterScene))
        {
            MasterScene = masterScene;
            LoadMasterOnPlay = true;
        }
    }

    [MenuItem("File/Scene Autoload/Load Master On Play", true)]
    private static bool ShowLoadMasterOnPlay()
    {
        return !LoadMasterOnPlay;
    }
    [MenuItem("File/Scene Autoload/Load Master On Play")]
    private static void EnableLoadMasterOnPlay()
    {
        LoadMasterOnPlay = true;
    }

    [MenuItem("File/Scene Autoload/Don't Load Master On Play", true)]
    private static bool ShowDontLoadMasterOnPlay()
    {
        return LoadMasterOnPlay;
    }
    [MenuItem("File/Scene Autoload/Don't Load Master On Play")]
    private static void DisableLoadMasterOnPlay()
    {
        LoadMasterOnPlay = false;
    }

    // Play mode change callback handles the scene load/reload.
    private static void OnPlayModeChanged(PlayModeStateChange state)
    {
        if (!LoadMasterOnPlay)
        {
            return;
        }

        if (!EditorApplication.isPlaying && EditorApplication.isPlayingOrWillChangePlaymode)
        {
            // User pressed play -- autoload master scene.
            PreviousScene = EditorSceneManager.GetActiveScene().path;
            if (EditorSceneManager.SaveCurrentModifiedScenesIfUserWantsTo())
            {
                try
                {
                    EditorSceneManager.OpenScene(MasterScene);
                }
                catch
                {
                    Debug.LogError(string.Format("error: scene not found: {0}", MasterScene));
                    EditorApplication.isPlaying = false;

                }
            }
            else
            {
                // User cancelled the save operation -- cancel play as well.
                EditorApplication.isPlaying = false;
            }
        }

        // isPlaying check required because cannot OpenScene while playing
        if (!EditorApplication.isPlaying && !EditorApplication.isPlayingOrWillChangePlaymode)
        {
            // User pressed stop -- reload previous scene.
            try
            {
                EditorSceneManager.OpenScene(PreviousScene);
            }
            catch
            {
                Debug.LogError(string.Format("error: scene not found: {0}", PreviousScene));
            }
        }
    }

    // Properties are remembered as editor preferences.
    private const string cEditorPrefLoadMasterOnPlay = "SceneAutoLoader.LoadMasterOnPlay";
    private const string cEditorPrefMasterScene = "SceneAutoLoader.MasterScene";
    private const string cEditorPrefPreviousScene = "SceneAutoLoader.PreviousScene";

    private static bool LoadMasterOnPlay
    {
        get { return EditorPrefs.GetBool(cEditorPrefLoadMasterOnPlay, false); }
        set { EditorPrefs.SetBool(cEditorPrefLoadMasterOnPlay, value); }
    }

    private static string MasterScene
    {
        get { return EditorPrefs.GetString(cEditorPrefMasterScene, "Master.unity"); }
        set { EditorPrefs.SetString(cEditorPrefMasterScene, value); }
    }

    private static string PreviousScene
    {
        get { return EditorPrefs.GetString(cEditorPrefPreviousScene, EditorSceneManager.GetActiveScene().path); }
        set { EditorPrefs.SetString(cEditorPrefPreviousScene, value); }
    }
}