Hmm, something where you cooperate against computer baddies would also be good. Anyone have any ideas there?
You can do this in Super Smash Brothers--up to four players/CPU characters, set up teams any way you like, and the CPU players can be assigned a skill level 1 through 9. I think you can also do this in any of the Mario Kart games, but I'm not sure.
And if they're hurting for exercise, there's always the home version of Dance Dance Revolution. They'll *have* to stop playing when they pass out.