You already have two of the best DS games in Mario Kart DS and NSMB, so I'll throw out a couple great DS games that either my wife or I (sometimes both) have played and liked. Some are great singleplayer, while some are great multiplayer games, and of course some are both....
* Mario & Luigi Partners in Time (single) - this is a GREAT single player RPG for the DS that really puts the dual screens to good use through some really neat puzzles and lasts surprisngly long for a handheld RPG. If you're looking for a really good RPG on the DS to hold you over til FFIII, this is the one to get.
* Yoshi Touch and Go (single) - this is, IMO (and my wifes) one the BEST, and most underated, and overlooked game for the DS. It is a platformer with a twist (the twist being the touch screen is used extensively and isn't just tacked on functionality) that has delivered countless hours of fun for my wife. While I really enjoy the game, my wife absolutely ADORES this game and has played it for at least 100 hours, and is now way better at it than I am (which has never happened before 😉 )
Princess Peach (single) - this is another good platformer that IMO is somewhat under-rated in the media. It isn't as good as NSMB, but is deinately worth the price of admission
Tetris DS (single + multi) - It's old school Tetris w/ a couple of new modes of play, a MUST HAVE for any DS collection. Need I say more?
Pokemon Trozei (single + multi) - This is another game that my wife and I both really enjoy that has gotten somewhat of a bad rap from the gaming press. While it certainly isn't the deepest puzzle game out there, it will none-the-less provide hours and hours of fun. Gotta catch em all 😉
Advance Wars DS - The best strategy game on the DS hands down. If you are into strategy games you should give this a shot, there are literally hundreds of units to play around with.
There have been many other games we've tried and have liked, but the ones listed above are the ones we really like. IMO, they are all must haves for any DS collection, enjoy.