I didn't see that these had been mentioned already, but they probably have since they're both fantastic games. Mario Kart DS and Advance Wars: Dual Strike are both fantastic. The former is classic Mario Kart with a hell of a lot of extras and fine-tuning, making it right up there in contention for the best game in the series. And Advance Wars is maddeningly addictive turn-based strategy that's just as fun with friends (via one DS, at that) or simply through the campaign. I highly recommend both of them.

Ah, I just looked at your post again and you said you got Mario Kart. Now get Advance Wars.