Ask her .. she would probably be pretty choosy about what the theme or venue should be. Teens are pretty specific about what they want and don't want. Better to make it HER day and let her choose.
You can give her "limits" with it .. but she should be the one to decide.
Slumber party with her best friends. That's always memorable.
I think setting up a small stage & having everyone perform a talent, or karaoke or even musical instruments if you have them. That would be fun. Theyll pretty much entertain themselves after about 2 hours tops. OR, you could call on your friends & do a show for the guests. Have fun.
Make plans that include social network connections, and even letting them make a "Second Like" charactor. Really at that age the social media and texting is all they want to do.