Pokémon could be fun for multiplayer, but it takes some time investment for both parties involved. There's a multiplayer mode on New Super Mario Bros. DS which is sorta fun, but it gets old fast. I've also played Kirby Super Star Ultra on multiplayer with my girlfriend, but its also a bit limited (you have to use the 1Ps screen to see whats going on.) Advance Wars could be fun to pass back and forth, but its intense strategy, and girlfriends might not like that.
After all that, I'd say Mario Kart is still the best. It's a blast!