I don't have a teenager (yet, he's almost 13), but at Nassau, I have heard a very popular excursion for teens is the Teen Junkanoo Jam. This is just for teens, but it appears to be only for a few hours.
There is also one at CC called the Wild side for teens-snorkeling,biking and kayaking.
I was looking at my booklet from last year, but I think they still have them.
I would also imagine that any excursion that involves being active such as snorkeling would be good. With teenagers, you have a lot of choices. They also have parasailing and a banana boat ride at CC. The whole family could do that together, but I have heard to bring goggles because the water really gets in your eyes.
Have fun!