For Him:
Legend of Zelda Phantom Hourglass
Pokemon Diamond
Professor Layton
For Her:
Nintendogs
For Both:
Mario Kart DS
Animal Crossing
New Super Mario Bros
Phoenix Wright 1 2 3/Ace Apollo (even moreso, but not exclusively, if either of you enjoy reading as well as gaming)
Elite Beat Agents
When i think about it.. most good games can be played by both you and her