Has she ever been to a character breakfast? My DD is also 15 and she loves the little Mermaid still!! We are doing a few character breakfasts as we have never done one before to add a little magic to our trip.
Another thing we are doing and I know this will sound silly but we are going to eat mickey shaped foods like mickey waffles etc. We started a Mickey shaped picture series at DL and are going to continue it at our 1st trip to WDW. So, we want pictures of us with Mickey waffles and different mickey shaped things. We already have pics with mickey light up balloons on NYE and mickey pretzels.
The trip is what you make of it. Find something special that you can carry a family tradition going forward if you continue to visit Disney parks. This will make her feel special and a part of your family.
What wonderful parents you are to take your DD to Disney for a surprise. She is lucky to have found such wonderful parents.