Advance Wars DS (A classic. One of the best tactic strategy games ever made)
Castlevania Dawn of Sorrow (This one turned me into a Castlevania fan. Probably my favourite DS game ever)
Castlevania Portrait of Ruin (Not as good as C
oS, but still very enjoyable)
Donkey Kong Jungle Climber (Very decent platformer)
Dragon Quest: Rocket Slime (Looks childish, not too difficult, but for some reason I enjoyed it all the way from beginning to end)
Elite Beat Agents (Everybody loves EBA)
Kirby's Canvas Curse (Awesome concept, equally good realization)
Kirby Squeak Squad (Easy to beat platformer, but still kept me occupied for a while)
New Super Mario Bros (Short, but very dynamic platformer)
Ouendan 2 (Everybody loves Ouendan)
Phoenix Wright 1,2,3 (They're all good)
Pokemon Diamond/Pearl (Got bored with it eventually, but still spent countless hours on it)
Sonic Rush Adventure (Did not like Sonic Rush too much, but Adventure one is good. Too short though)
Theme Park DS (I remember playing TP on Genesis. Not only it holds nostalgic value, but is pretty fun too)
Tony Hawk's Proving Ground (Plays like THPS and holds a ton of replay value)
Wario Ware: Touched (Great for a quick dose of mindless fun)
Zelda Phantom Hourglass (Zelda)