Well I will tell you what I did for mine, it was pretty inexpensive! But I've never been high maintenance lol. I got permission to use the youth rooms at church (but this could be done at home too), my mom made cupcakes and snacks, my friends came and I had games planned (like hide and go seek in the dark), we had music playing, dancing, and jsut general hanging out. My dad and his friend sat in the other room, gave us our space, I don't think I saw him but once! It was very fun and very simple =)
Another idea is a scavenger hunt, but this requires some parent involvement and more time/effort! Divide into 2 teams (or more if there's a lot of kids) and give them each the same list and either a digital camera, poloroid camera, or video camera. Then set a time limit of a couple hours and whoever can document the most items on the list in that time frame wins a small prize (goodie bags maybe?) A friend of mine did this for his birthday (although we were 18 so we just drove ourselves) and it was a blast! We used video cameras and it was hilarious watching them afterward! Some ideas for the list are sitting in a phone booth (hard to find nowdays!), riding a kiddie ride outside a store, eating a McDonalds hamburger, sliding or swinging at a park, climbing a tree, rolling yourself in toilet paper, etc. Just remember that you can't take a camera in a store or restaurant (we got in trouble for this not knowing lol)
One more idea if she wants it not co ed is a slumber party/makeover party. Buy a few cheap makeup sets, nail polish, lotions, pedicure/manicure stuff, etc and have them make each other over, then have them bring clothes and have a fashion show. Have categories like most creative, funniest, most beautiful, futuristic, etc and give out small prizes for a winner in each category
Hope this helps, good luck!