Do you have a
Disney store near you? Another thought would be along the scavenger hunt lines....you could print out clues with disney characters on them, hide them throughout the mall, etc., you may have to get the help of a friend of hers for this, and get her to the mall or wherever in town, that leads to the Disney Store (or any other way you want to tie Disney into it) at the end with you there waiting to ask her to Prom...DD's BF did a simlar thing for their Prom...altho his focused on sentimental stuff/places they like etc., but DD LOVED it....
Or you could rent her fav Disney movie, and ask her to put it in the DVD player, when she opens the case you could have a note in there,
so many ideas...what is her favorite character or movie?
Given it is Prom a natural thought is a ball like Cinderella, but there are so many other more creative paths to take....you cuold tie Mulaninto a fortune cookie that has you askking inside as the fortune,
I imagine she may be a Jack Sparrow fan..maybe a compass that point to the direction of gong to Prom with you?
I am sure you willl come up with a great idea, have fun!