I'm thinkin Mario Kart and New Super Mario Bros.
I've got both and they're pretty good. The thing about New Super Mario Bros is that it's meant for people who are new to Mario as well as veteran players. As a result I've got like 80 lives. It's still very fun, but you'll probably enjoy it so much that you don't put it down and finish it in a few days.
Collecting all the star coins and completing the alternate endings to the levels adds a bit more life to the game. The mini-games are fun but most of them get boring fast.
Mario Kart is really fun. One of the best Mario Kart games to date.
There are attachments for the DS to let you use MP3 Any type of video file and exe.
DSLINUX FTW.
Here is what i am getting. http://www.consolesource.com/ecomm/catalog/ Max-Media-Launcher-p-2444.html
Its size is the same as a normal DS cart