On our last cruise, our friends with daughters aged 16 and 18 went to Atlantis and did the tour with the aquarium, then they went shopping. They had a great time. We did the Blue Lagoon Beach day with our 9 & 12 year olds...we snorkeled, which wasn't the greatest and ate and that's about all. On our next cruise we are going to do one of the snorkeling excursions like Caribbean Queen, and we have people going with us who don't snorkel, just want to enjoy the experience of being out there.
Without any young kids on
Castaway Cay, I'd enjoy the adult beach, definitely do the Banana Boat, and maybe even Parasail. Otherwise, I'd just sit back and enjoy, do a little swimming, relax at the bar, watch for the characters.