DS Games:
If you like rail shooters, Touch the Dead is good.
EBA/Ouendan for beat games
Lock's Adventure (or was it Lock's Quest) for an interactive Tower Defense (like you walk around and fight while building castles)
Ninja Town (iirc) is a fun little Tower Defense
Desktop Tower Defense is a cheap but fun Tower Defense
Layton games if you like puzzles
Pokemon Ranger is decent too
There's also Chrono Trigger remake for the DS if you're into that
Rune Factory is a good RPG-based Harvest Moon
Final Fantasy remakes exist too
I could go on but I lost like half my save files because my external hard drive went crazy one day :(