get into Homebrews:
some drawing programs(Colors! and DSnote),
if you want to make simple flip style animations: FlipBook
some emulators (nesDS, lameboy, jENESIS DS, snameulDS),
some music player/editor: GameMelodyOratorioDS, NitroTracker (for chiptunes),
some greats FPS games that have great ports on DS (QuakeDS, Quake2, DSCENT),
if you are and old adventure's fan try SCUMM DS(for alot of oldie adventure games, namely, Indiana Jones and fate of Atlantis, Monkey Island 2, Flight of Amazon Queen, etc)
some random homebrews: PocketPhysics, PuzzleManiak, DSLinux, PlatdudeRetro
if you are a fan of Rogue-like games, try: ASCIIWARS, NetHackDS, POWDER 102,
those are the homebrews currently on my DS, you can find them all in homebrew section of this site:
http://gbatemp.net/index.php?dlcategory=1
Thanks to Urza for his hard work on introducing homebrews.
if you want more, tell me and I can find my topics that people voted on best homebrews.