DS + Mario Kart DS + New Super Mario Bros. + Phantom Hourglass + Metroid Prime Hunters = Endless enjoyment.
And as long as you have it on the low brightness setting with no sound, you'll get a solid 15-20 hours of battery life, no joke. I recommend you get the Zelda Bundle Pack that's out now. $150 for the system with an awesome design and Phantom Hourglass, one of the best handheld games ever made. Then get Mario Kart. That game is addictive, and has a really good online mode.
BTW, before you ask, best baseball game on the system is MLB Power Pros.