My wife and I have the most fun with the DS.
Depending on the games you enjoy...
Pokemon Diamond/Pearl are great -- if you enjoy that kind of game style. Dragon Quest Monsters: Joker is great too.
SimCity DS was $20 at Best Buy on Christmas eve.. if it is a sale or a drop, that could be fun. Elite Beat agents is great. Rocket Slime is great...
If you like RPGs, Mario and Luigi: partners in time is fun. Brain Age 1/2 are nifty as well.