Although theres plenty of other things you can do:
Play emulators-You can emulate gameboy color, nes, snes, and a few other old systems.
Play music - Add songs and turn your DS into an MP3 player!
Watch Videos - Put videos on your DS such as movies (probably WAY too big to fit a FULL movie =O)
Play DS games - Launch DS roms straight form the card! (WARNING: DOWNLOADING ROMS IS ILLEGAL UNLESS YOU OWN THE GAME!)
Ported Games - There are a few old games that are ported to the DS such as: WOLFENSTEIN 3D! (Single handedly started the FPS gaming!) DOOM 3D (Same engine as Wolfenstein) Quake! (New engine based off of Wolfensteins original code) [People are modding Quake DS to be like halo! Its called iHalo DS! Its a great little game.
Custom Games- People make cool homebrew games ALL the time such as little racing games, simple shooters, pacman etc.
cool ;]