Nintendogs is quite fun, but slowly loses its fun factor. (7.5
Animal Crossing wild world is great, and you can do quite a lot, if you can access WiFi, it is A TON better. Still good nevertheless, I still play it quite a bit. (8.7)
Super Mario 64 DS is pretty darned good, it could take a few months to get all 150 stars, depending on your skill level. I loved it, when I got far though, it wasn't as much fun, but there is always the minigames. (8.0)
Metroid Prime Hunters is INCREDIBLE! If you get WiFi, this game is... *kneels down* "GOD OF GAMES" lol, not really, but it is incredible. Voice chat with friend, add rivals on WiFi, good story mode, 7 hunters and 7 modes! With 25+ stages! (9.4)
I dunno about Tetris DS, although I heard it was really good.
Summary of games to try...
Kirby Canvas Curse
Super Mario 64 DS
Metroid Prime Hunters (Strongly suggested)
Nintendogs
Animal Crossing Wild World