It really depends on her personality, her likes and dislikes. Does she enjoy crafts or baking? You could have the girls make a cute craft (I'd avoid the cheesy foam ones; spend a little extra and get something gift-worthy), or have them make and decorate cookies. Or decorate canvas aprons for the craft, and then wear them while making cookies or another fun recipe, such as personal pizzas or make-your-own sundaes. Or if they are more experienced in the kitchen, select a slightly more challenging recipe, but one they can still accomplish with attractive results.
Or maybe she and her friends like board games; online, you can search and find free instructions and rules for some new ones and/or classic ones to print out. Make the supplies ahead of time, or let them create/decorate their own version and then spend some time playing them.
If she likes scented lotions, doing her hair, and painting fingernails, like others have suggested, you could make it a "salon"-type theme at home. No need for expensive products, either--have the girls each bring something from home (a nail color or a lotion to share), or make your own products as part of the fun (find recipes online or in books borrowed from the library). You could include "mud" masks and make it more of a "spa" theme, letting each girl take a turn being pampered by the others--hair, skin, nails, etc.
If she likes formal things, a grown-up-style tea party might be the way to go--bring out the good dessert dishes and a pretty teapot (you can put something in it besides tea if necessary) and serve finger sandwiches, hors d'oeuvres and fancy tartlets. There's just something about bite-sized servings that seems to appeal to kids of all ages. ;-)
My daughter is more of a tomboy, but for her birthday, she and her friends enjoyed making jewelry--of course, not using the cheap little plastic beads for kids, but the pretty glass, semi-precious, and metal ones that I have used myself. My daughter selected most of the beads herself--within reasonable spending limits, of course. They also played tag out in the yard for about 30 minutes, and then came in for cake, ice cream, and gifts. After that, they pretty much just chatted and giggled until it was time to go home. They could have played other games that I had planned (Twister, Crazy Eights, and Girl Genius: The Works, to name a few possibilities), but they were having fun as they were, so I didn't bother to interrupt that. :-)
HTH, and Happy Birthday to your daughter!