Well, to get Mario games out of the way, Super Mario 64 and Mario Kart DS are great titles. Mario VS. Donkey Kong is another good one.

Try out the Castlevania games if you can find them. They're great adventure titles.

Star Fox DS is good in my eyes, but a lot of people really hate it.

Edit: Oh, stupid me. You're already getting Super Mario 64 and Mario Kart. Well, get New Super Mario Bros. as well!