I prefer to do a gift baskets rather than special gift. I get lots of ballet themed small things of the net or ballet shops (i.e. ballet socks, CD's, hair pins, ballet tights, book, dance accessories like ribbons, personalized bottle, small diary to keep her notes, dance warm ups, leg warmers perfume..etc.) organize it nicely into lovely basket wrap it all up into cello bag add big ribbon bow on top and ...voila.
Good idea for some would be perhaps subscription to dance magazine or you can’t go wrong with theater tickets.