For DS, you need Kirby Canvas Curse and at least one Advance Wars game. And I mean, NEED. They're both effing awesome.
You also seem to like platformers, so I'd check out some of the more overlooked ones on the system: Kirby Squeak Squad, Super Princess Peach, and the first Sonic Rush, for example. If you liked Castlevania: Portrait of Ruin, it might not hurt to pick up Dawn of Sorrow, either.
Finally, if you liked Hotel Dusk, grab Trace Memory. It's a short, flawed game in comparison, but it should be really cheap now.
As for GBA games: Try Drill Dozer, Rayman Advance, Zelda: The Minish Cap, the first Mario v. Donkey Kong, Lady Sia, and the three Sonic Advance games. If you're into strategy games, you may also like Fire Emblem and Tactics Ogre: the Knight of Lodis, though expect to pay out the wazoo for either. Finally, Mario & Luigi: Superstar Saga is a fun, shortish Paper Mario-style RPG with some of the best humor writing I've ever seen in a game.
"'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
