Im going to suggest some PS2 games because other than DS, thats what you seem to have:

Smackdown vs Raw 2006 (good online or offline)

Smackdown vs Raw 2007

Smackdown Here Comes The Pain

Ratchet Deadlocked (Two player Co op story)

Ratchet Up Your Arsenal (Offline Competitive/Co Op, but you need multitap for up to 4 people, or online for Co-op or competive for up to 8 players)

Some game thats titled something like "Baldurs Alliance" or something, I cant remember, but it is a co-op as well

Nhl 2k5 (Its an old game, but 2k5, and past is real fun)

Ea's Nhl games (2004 is probably the best one)

NBA Street Volume games are real fun (I prefer Street VOl. 2 but thats my bias)

Basically any sports games

If you have online, choose Burnout (I like 3, but again, my bias, or Revenge is fun too) Offline you can play party crash, which can be  co-op or competitive. Online on Burnout 3 and Revenge is alot more fun though

GTA: San Andreas has a two player, though its not the best multiplayer Co Op out there

Tekken Tag Team, if you can find that game

And thats about all I can remember, so check them out cuz i took along time writing this out lol