Verified answer

That depends on what your into. There are a ton of great DS games out there, and if I listed off every one of them, I would be here all day.

I'll list a few of my favorites though.

Mario Kart DS (well, you already have it, but if you have the friends to play with, this game can last a LONG time)

Kirby Super Star Ultra (Fun, easy at first, challenging towards the end, and it can get addictive)

Sims 2 (The one where you run a hotel in strange town, I would have never gotten this game for myself, but I'm glad my mom bought it for me, it has a strange quirkiness, and is one of the most original games I've seen)

WarioWare touched (first night having this, I stayed up all night playing it)

Pokemon Diamond/Pearl (still not like the old days, but I've been hooked on Pokemon since Red and Blue)

Well, I hope this helps.

Rayman Raving Rabbids 1

Rayman Raving Rabbids 2

Tornado

Shrek Smash N' Crash Racing

Animal crossing Resident Evil and Guitar Hero are my favs