get a mario kart copy and you will be a favorite among your DS toting freinds for download play, also consider both the Mario 64 rehash and the New Super MArio Bros. both are excellent revivications of classic masterpieces, and the DS is able to keep up admirably with the graphics you'll remember from the N64 (indeed it improves upon them in MarioKart)
The Sims 2 is short lived but addictive while you play it.
I kill time waiting for the bus/chow line/etc. with a charming little puzzler called Polarium.
Metroid Prime: Hunters is a solid addition to that franchise, and also sufficiently hectic in multiplayer, the graphics can seem a little grainy though.
Lastly, Urban Asphalt GT 2 is decent racer that has only two failings: first, that is no difficulty of any kind in achieving first place in every race for the first half of the career mode, and the motorcycles have terrible manueverability in comparison to the track contours.
Now what I'd like to see on DS is an extreme G installment. I finished XGRA and I'm drooling for the next dose of supersonic motorcycling mayhem.