Is this going to be an all girl party?  Also, when you note that you have a budget of $100 for an activity is that $100 strictly for an activity or does it need to include food, favors, etc.?  Finally, do you have a time frame for this party... 2 hours, 3 hours, more?

It usually works out well to build out activities and a party theme around a favorite something of the guest of honor, whether that be a movie, a food, an activity, a song... anything really.  Do you have any favorites?