Well, I hate to say it but I DM (and yes, it's fun.) I also play, though (one of the other players DMs also.)
What I do as a player is level him/her up as far as I can, usually in different ways. I like to see just how many PrCs I can hit and what skills, feats, or classes I need to get to each one fastest. I also like to see what that character would be like if he just stuck to one of the classes he has. For instance, instead of becoming any Rogue-inspired PrC, it is nice to quickly progress in just Rogue for the upper-level benifits. This is just an instance and not always the way I do things, or the way they should be done. Sometimes it is cool to max out in Rogue but take just a few levels of a spellcasting class. You could make a "good" assassin this way. Other class/race combinations can benifit the same way of course.
While alot of this seems rather stat crunching, the most fun I have is finding non-game mechanic reasons to do so. What makes my character a hero and why would that character choose the path he did? At some point it might be nice to run a character who is just not that in tune with maximizing his life (or stat crunching in game mechanic terms.) But for now, I will play the hero.