We've tried a number of games and these are our favourites:

2 button:
ksmiletris, by far the most successful I've used, fun for everyone.

4 Button:
ktron, the racing game, but slowed down, email me for the tar.
kpacman, I've yet to find a succesful cheat for our clients.
kasteroids, one client on the fire, another aiming.

kpat, too difficult for many. 'idiot' is a check on suit & numeracy.