I would say a movie/karaoke/singstar slumber party where they are left alone to have fun (no mum, dad, brother or sisters butting in all the time)
Supply them with loads of girly type things such as nail polish, facial masks, brushes, hair ties, make up (if you allow that) and let them go crazy doing make overs on each other. Perhaps let her have some control of the camera so she can get some photos.