TWEW (the world ends with you) - good rhythm to start with, and it gets more hectic, especially if you try juggling both screens at once
any game named warioware - basically a series of "twitch" minigames. also quite good as a party game, according to most people
zelda - despitew the zelda games being famed for the puzzles, the series is a fun action game with some RPG elements thrown in
most DS FPS games, notably metroid prime: hunters - metroid in the DS is a well-made, action-packed game that makes the FPS genre really shine on DS (hand cramps after several minutes of play, esp. in multiplayer might become a problem, though)