A couple other ideas I didn't see listed here yet:
1. Rent a boat to view the fireworks at MK or Epcot. We've done both. If you request it, you can have the boat decorated with a sign and balloons. We surprised my daughter with a boat trip after dining at 1900 Park Fare - strolled over to the boats and there it was decorated to the hilt. We cruised around the lake and had an amazing view of the fireworks. Same with Illuminations at Epcot.
2. Have the Disney Florist decorate your room for your arrival or after arrival if her birthday falls within your trip.
3. Engage a local company to do something special one day while you are in a park. We hired a company to do a special Scavenger Hunt for my daughter around the Magic Kingdom a few years ago and it was amazing. They customized the hunt for my daughter based upon her interests and the milestone we were celebrating. While 16 may be too old for that, my guess is that if you found the right company to do this they could make it more age appropriate, and who doesn't love a little pixie dust? We had the best time and my daughter was absolutely amazed at the "magic" this created.
But the best suggestion I have = scour these boards! That's where I've found all the ideas I've done to make each trip special. There are so many great suggestions here. Have a great time planning!!