Make certain to get the birthday pin and make notes on any reservations. Sometimes, she may get something a little extra for wearing the pin.
Would she wear/appreciate a tiara (bring from home)? send them to the pool and decorate the room (again stuff brought from home) even doing the windows/door on the inside.
Assuming this isn't her first trip, could you make a then and now book (pictures of her at a younger time, same pose, character now)