Animal Crossing is fun for about a month, after which you realise you've seen just about everything, and it wasn't that much.
Have you tried a dungeon crawl game, like Diablo 1/2, Dungeon Siege, etc? My wife and I are playing through Champions of Norrath (PS2) right now, and Baldur's Gate: Dark Alliance before that; she likes it for the co-op play. She never cared much for The Sims, but she likes the Monster Rancher (PS, PS2) series.
If your GF liked Age of Empires, maybe you should try another RTS...