Isn't Brothers in Arms DS is multicart only? I could be wrong.
Me and my cousin rely heavily on download play for some multiplayer fun since he doesn't have a flash cart. These titles are favourites..
Ridge Racer DS - A fast arcade racer with impresive download play content. My brother loves it.
Bomberman - Not Bomberman Touch. The first one has more multiplayer modes, although Bomberman Touch has the mini-games.
Konami Classics Series - For some retro gaming, Contra, Rainbowbell and Track and Field are pretty good.
Tetris DS - Yes it has download play. Push mode is fun!
New Super Mario Bros.
Zoo Keeper - Probably won't entertain the guys for that long, but the girls love it to death.
Clubhouse Games - Another favourite. Bowling is hilarious!! xD
Jump Ultimate Stars - Rather limited download play. Multicart would be better.
Brain Training - Ever seen someone finish 20 math questions in 10 seconds? =D
Legend of Zelda: Phantom Hourglass
Tingle's Balloon Fight
Heard Meteos is great too, haven't tried it personally.
Hope that helps! =D