Is Hannah Montana still in? If so, maybe a H.M. themed party?
Girls that age usually like to dress-up and wear "makeup," so maybe a "Houston's Next Top Model" themed party. You could decorate the party venue (i.e. your den) like a salon and give the girls "makeovers." Spend about $25 on nail polishes, hair pretties, lip glosses, blush, eyeshadow, sponges, makeup brushes, etc., and treat them to a day of beauty. You could make a runway (a hallway in your home) and have each girl walk down it while you take photos. If you have a photo printer, you could print the photos on the spot and then let the girls make cutesy frames (or maybe they should do the craft before the makeovers).
If money's no object, you could surprise the girls by having them picked up in a limo and taken somewhere fun.
**My mom/stepfather own some beautiful limos, by the way. Email me if you (or anyone reading this) is interested =).