Well, obviosly having a working drivetrain of some kind is critical. It's not interesting if your robot just sits there.
Since you're not nessesarely working under the parameters of FIRST, you have a lot of flexibility as to what you can do, it also depends on the audience you're trying to get interested. We found that our 2006 robot (Aim high game, where you shot small nerf balls) was very popular amongst visitors (especially children).
I guess I would reccomend making a robot that moves something around somehow. Just driving is a bit boring. Launchers go over well, but are not nessesarely good for small spaces, perhaps an arm to lift some object?
Just some thoughts, feel free to be creative, and since it's not going to be in an actual competition, do whatever you want with it (but keep it safe).
