I second Stunt Car Racer.
The Lotus racing games. I think racing games (or basically anything where you can use the tilting as a steering device) would work fairly well. Plus we might finally get some decent draw distance!
K240. I would buy that, especially if it let you pick between new and original versions of all the structures, and also let you build without threat of alien attack.
Any and all point and click adventure games! I know Simon the Sorcerer is already on it, and I can run SCUMMVM to get some of the other best ones, but I'd like to play games like Nippon Safes Inc., Universe and the like.
Mega-lo-Mania. I've played the open source version on my SIII and it worked really well.
Marble Madness. As if that game wasn't hard enough! But it does seem to lend itself nicely to tilting.
Defender of the Crown!
