I don't know if you own a PS2, but I would still say it is one of the greatest systems of all time. Hundreds of great games throughout just about every genre imaginable. And yes, games are usually very cheap. For 100$ you could get a plethora of used games that would make you very happy.
Wii might be good for you too. But not exactly for Wii games, but for Gamecube games. If you haven't played gamecube before, you can pick up most of the killer apps for it somewhat cheap. Anyway, you said cheap, and fun, so these 2 systems come to mind.