Massive seconding for The World Ends with You - probably one of the few decent square games released in the last decade as well as one of the best games for the system.

Others:
Any (but only one) Yu-Gi-Oh game (nightmare troubador reccommended due to the smallest banlist)
Megaman StarForce (comes in three different flavours, like pokemon but fewer differences)
FFIV-DS (fanciest version of, consider skipping)
Advance Wars - Days of Ruin
Lock's Quest
Henry Hatsworth in the Puzzling Adventure - This should be your top priority.
Super Robot Taisen OG Saga - Endless Frontier
FF- The 4 Heroes of Light (some light jrpging for you)

If you want something more casual, try one of SEX's Space Invaders Extreme. Alternatively, my DS-flavoured sudoku fun of choice is Sudoku Gridmaster (responsible for at least 25% of my logged pooping hours since 2007).