In San Juan, a tour of old San Juan is very interesting. There is a botanical garden that is really nice. If you are going to be there a while, the locals go to the beach in Ponce, which is on the other side of the island. A tour of the Bacardi factory is fun and tasty. In St. Thomas (here I go again) take the ferry to St. John and a cab to Trunk Bay. It is a beautiful white sand beach (like baby powder), with warm, crystal clear blue water, and a self-guided snorkeling tour about 50 yards from the shore. There is shade available, a snack bar, lockers, fresh water showers, a couple of shops (shop in St. Thomas), and a place to rent snorkel equipment. If you stay on St. Thomas, a catamaran sail/snorkel to Buck Island is fun. St. Thomas and St. Maarten are good for shopping, if you're into that. We did an America's cup race in St. Maarten that was a real blast. A bunch of cruise passengers crewing a racing yacht, what a hoot!! Best wishes and have a wonderful cruise!
Never been to San Juan, sorry. We did a catarmaran sail/snorkel to an island while in St. Martin once. The island was called Tintamar. The snorkeling was really great. We did however take the morning excursion, they had a choice for morning or afternoon. If you consider this, the afternoon might work better, in the morning the water was a bit cold, (Early Feb. is when we were there). Like Orient Beach, there is nude bathing permitted, and like Penny says, these folks should be clothed, some of thier bodies were falling fast, I'd advise wearing a hard hat just in case! The excursion is offered by many different cruise lines including Princess, Holland America & Carnival to name a few, I don't believe Royal Caribbean offers it but they should. The excursion was called "Golden Eagle Catamaran Sail to Tintamar". Last time I went to St. Martin I rented a car and drove around the whole island (I'm not going to advise this) looking for the perfect beach, one better than Orient. Didn't find it and got lost all day. We've been to St. Martin 5 times so far and I have to say that the Golden Eagle to tintamar island was the best. Also, since you're going on Carnival, they may also have another sail/cat/snorkel excursion from St. Thomas. We did one there on a ship called "Castaway Girl" to an absolutely gorgeous secluded beach on St. John. It was the best best BEST sail/snorkel we have ever been on. I think Carnival might offer this excurion since Holland America has it and they seem to have the same excursions (probably because they're now owned by the same company.)