I don't know, everyone has a different preference with games, it's not just girls like one category. There aren't any multiplayers for Wii that stand out, just yet so I'd think of getting GameCube games.
Well, my fiance enjoys playing Gears Of War with me on co-op (though she gets killed pretty often), and other Arcade games that have multi-player support.
Can't really speak for 'all' girls since everyone has their own taste in something, but multi-player games are usually the best way to go when with a loved one. Like Frogger mentioned, Gamecube games would be a good choice, or you could just stick with Wii Sports and see if that works.
my g/f and i play MK armageddon. she agreed to get really into smash bros with me when i get it too. also i plan on playing zak and wiki with her when i get it, becase the 2nd player can move a pointer around and circle stuff, so she can tell me what (she thinks would be good) to do.
Mario Party (Any version)
Super Paper Mario
Wario Ware Smooth Moves
My advice for you would be to go to a video game rental store such as Blockbuster or Rogers Video and have her pick out a game or two that she thinks will be fun.
Multi player games would probably be your best bet. Anything Co-op is also a really good choice.
Seeing as how i'm a girl myself, i can speak from experience. I don't own a 360, but whenever i go over to a friends house who has one and we decide to play, i like to warm up by playing some Co-op Halo or Gears, you know, help me build up my confidence. Once i start feeling more comfortable, then bring on the Online challange against other people.
Boxing on Wii sports.
Get her all angry at you, then you can have a "make up" make out session.
Or, you can let her win and then she'll "console" you for the loss.
Either way you'll have fun playing and afterwards.