Yoshi's Island DS is the sequel to one of the best, most beautiful 2D platformers ever made. And it's pretty good in its own right.
Either of the Advance Wars games are great if you like strategy games, or even if you're a newbie to the genre. They're two of a very short list of games in the genre that both newbies and veterans can enjoy.
If you like Rogue-likes, the best ones on the DS are the Pokemon Mystery Dungeon games. You might also want to check out the Etrian Odyssey games if you like challenging RPGs. Avoid the Izuna games like the plague, though.
Lessee... if you like survival horror, you might like Dementium: The Ward, if only because it nails DS FPS controls. It's also decently creepy.
Also, are you looking for GBA recommendations as well?
"'Casual games' are something the 'Game Industry' invented to explain away the Wii success instead of actually listening or looking at what Nintendo did. There is no 'casual strategy' from Nintendo. 'Accessible strategy', yes, but ‘casual gamers’ is just the 'Game Industry''s polite way of saying what they feel: 'retarded gamers'."
-Sean Malstrom
