Well if you're not counting Super Smash Bros. Brawl in your list of Mario games, buy it. Girls love Brawl.
And definitely, definitely buy a music game. Find out what bands she likes, and buy accordingly. If she's a Beatles fan, she'll love The Beatles: Rock Band. Or if she's not up for music games, get something fun and cooperative like Boom Blox or Zack & Wiki. And if she loves shooting ****, Call of Duty: World at War is the way to go.