It all depends on what you like to do, really. After you beat the game 100%, you can still play the mini-games and just mess around with people.
This does become quite boring after a while, however (like any other game). So what you can do is start a new file or whatever. What I did was make a few save files for a few select missions that I liked, and whenever I feel like it I'll go back and replay them (if Bully had a mission replay feature, that would make this game perfect).