I am currently making a simple mobile game. I think I can make the GUI interesting if I do it all on one scene (i.e. Main Menu, Options Menu, and the actual game will all be on one scene). The biggest issue I have with this strategy is that, by the time I am done doing this, there will be tons of objects just lying around outside of the view of the camera.
Is this going to affect performance?
if you do not have occlusion, and probably even with that, yea it would affect performance. you could be right in having what is known as a ‘MASTER’ scene and then using the Application class, you can do things like ‘LoadLevelAdditive’ where it adds scene data from other scenes to your current scene. Basically to better answer your question, I recommend you take a comprehensive look at the Application class in the unity docs.
I’m aware of several commercial Unity game releases that have made use of only a single scene. For example, Xiotex’s Blast 'Em and Mike Bithell’s upcoming Volume.
Rather than just being placing menu options etc. outside the camera viewport when playing the game, you should call SetActive(false) on them - that way they’ll be disabled and won’t eat up any CPU cycles until you enable them when needed.