This made for a lot of fun and a party favor at the same time. Break the girls into groups of 3-4. Let them pick a song that they will lip sync and perform a dance routine to. Let them practice and then video each performance. Video tape all of the performances and then make copies to give to the girls as a party favor/keepsake. We did this for my daughter when she was 11 and it was a major hit!
Leave them to it. They will always entertain themselves. Nothing worse than your mother / father trying to throw a bday party for you when your 13+ !!!
In general tween girls want to be grown up and glam. If you have a room in the house that isn't going to be used or your daughter's bedroom, get one of the tween's to help you on what dvds to get etc, set up a tv, dvd player, ipod dock, plenty of nibbles, food they like, maybe set up some nail polishes and make up so they can give each other manicures and or make overs etc, hide the scissors in case the make overs head in to new hair do territory! Or if you are near town and depending on their age, arrange to have them driven to the local cinema multiplex, watch a movie, have some dinner and then be picked up again.