If you're interesting in doing a little reading, then I'd recommend either:
Lone Wolf DS: A port of the 'choose your own adventure' books to the DS. Great user interface and so polished you'd be forgiven for thinking that it was a commercial product.
2 books currently adapted.
or
DS Visual Novel Reader: Pretty much what it says, and while Visual Novels might not be everyone's cup of tea, for others, the chance of playing games like Tsukihime or Fate/Stay Night on the move is a welcome option.
6 VNs currently adapted.
Other things worth checking out:
ScummVM DS: Probably goes without saying, but this is surprisingly overlooked. Once originally just a emulator for the SCUMM system that Lucasarts used to make their famous 'point and click' adventures, the project has expanded to cover games from other publishers as well. Plus, you can get freeware versions of Beneath a Steel Sky and Flight of the Amazon Queen to run on it from the site as well.
XRickDS: A port of the old Amiga game Rick Dangerous. If you're a fan of Indiana Jones or unfairly difficult games like 'I Want To Be The Guy', then you'll have hours of fun/frustration here.
Dscent (developer's link's down for the moment): A DS port of the old FPS Descent. Possibly more vomit-inducing than its PC incarnation, it runs better than most other FPS ports out there.
NesDS: An NES emulator for the DS. Doesn't sound particularly special, but the inclusion of a 'reverse' button is so enjoyable, you'll wonder why you ever played an emulator without it. Games that would normally require heavy amounts of load/save stating to beat become a lot more fun as a result, as you can just hold down the button to take back an ill planned move, 'Sands of Time' style.
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