Germany and Austria! Loved it! I have a friend who lives in germany (about 15 years now) and considers Austria the most beautiful place in Europe. Another friend married a man from Belgium and says the same thing. Both have been to every country multiple times over. We were there in December and it was breathtaking. I cannot imagine in the spring/summer.
She's a dancer and she loves history - particularly the WWII era. She does like museums but I wasn't sure if I should do an exotic location. She's not really a lay around the beach everyday kind of teen.
What about Greece? Lots of history, lots of stuff to do, and it's cheapcheapcheap once you get there right now.
Morocco is also incredible. Fascinating history, some culture shock, and just really different than the places she's already been. Turkey might be good for this also but I haven't been so I can't say for sure.
I think cruising is a good option! Sounds like she loves travelling and a cruise would hit many spots, plus she can relax during the at sea days, and there's plenty of fantastic shore excursions she can choose from depending on what she likes.
Caribbean can be as relaxing or active as she wants in regards to doing stuff like swimming with dolphins, etc or just hanging out in a beach.
European cruise would be educational and active with seeing all those historic sites.