I already did all i could before i met head on with the blatantly obvious barriers on what's possible which you will soon meet.
It IS endless if you have an imagination. You clearly don't. You are without a doubt the type of person that buys a game, completes it, and then takes it back to the store. The game has no replay value for you because you're boring and unimaginative. There are people out there, like me, that could play for hours on SP having fun and finding new things to do every day.
Like i said previously, i, with help from friends, got more out of GTA IV than most people could imagine. And i will be able to do the same with GTA V.
That's the thing, there are no barriers for me. I will always have an idea of what to do, and i will always be able to do it. I won't think of something that's not physically possible within the GTA V world, i'll think of everything that's there, and thing of things to do with it.