In no particular order:
- Elite Beat Agents/Ouendan 1/Ouendan 2 (get Ouendan 1 and 2 *after* getting EBA, unless you can read Japanese)
- New Super Mario Bros. (includes fun multiplayer modes too)
- Super Princess Peach
- Contra 4 (awesome, but super difficult!)
- Namco Museum DS (Galaxian/Pacman/Galaga/Xevious/Mappy/Tower of Druaga/Dig Dug 2, plus the awesome multiplayer-only download-play game "Vs. Pacman")
- The Legend of Zelda: Phantom Hourglass
- Kirby Squeak Squad (my son loves it and would recommend it highly, though I haven't played it yet - i looks like classic Kirby style gameplay, unlike Kirby Canvas Curse (which we don't own yet.))
I'd also give a mild recommendation for Meteos too, but truth be told, while I did play it pretty religiously for a while, after a while it just seems like most other "puzzle" games - how well you do seems as much due to luck of the draw than anything else. It's a good time killer game, though.