When I moved down with my folks I was 17 and we moved to Cayo. Things were very different back then and it took me a while to warm up to Belize. I eventual fell in love (w/ Belize) and I moved here permanently on my own a few years later.
As far as AC, tons of kids and teenagers abound. If not home schooling (I did), going to High School here is a great way to get involved in the local culture and to make lifelong friends.
I enjoyed travelling in the region with my parents, the Cancun area was a blast as a teenager.
Here on the island, SCUBA would be great. I wished I learned when I first moved here. Joining a sports team or dance group may be enjoyable.
For the most part, get her out and about, it's not too hard to meet folks while walking around town and on the beach.