Here's a nice game for your DS. It's Parallel Worlds: Combined, by strata8. It's a 2D platformer, which is basically a rehashed Mario game. Apart from the blatant Mario-ness of the game, this one actually offers features that can hold their own against the Italian plumber.
Parallel Worlds features a unique camera system, realistic AI, and some pretty catchy music. strata8 is welcoming suggestions for new levels and characters, so be sure to tell him about it and find your name on his next release's thank you list.