I finally bit the bullet and picked one of these up (a DS Lite) last week.
I picked up a copy of Electroplankton with it; that's the sound/music/a-life toy by Japanese media artist Toshio Iwai (who also designed the Tenori-On, which recently launched in London). It's rather fun, and good for creating textures and ambience. Some of the modes are more interesting than others; my favourite is probably Luminaria.