If you've got "one of those card things" then there's nothing stopping you downloading a load of stuff to try it. Obviously you can only legally download games you already own, but of course you know that and wouldn't be posting on here requesting illegal ROMs.
CoD4 is pretty good on the DS, as is World at War.
Mario 64, New Super Mario Bros and Yoshi's Island DS are all good too.
There's a zillion puzzle games that are good, from Tetris and Solitaire to Sudoku and Brain Training.
I really like Warhammer 40k Squad Command and the Advance Wars games too.
And you can't beat Nanostray 2 for some seriously hard 2D shooter action (with online scoreboards).