Hi Sarah5555, welcome to Cruise-Chat! Congratulations on your up-coming cruise! I hope you enjoy the Solstice, she's a beautiful ship. We were on her in December. There are tons of posts about your ports in the Caribbean Port Chat section, just a little further down the main page. You can use your "find" feature in the upper left hand corner, type in your ports and it will pull up all the posts made about them. You might find something really interesting.
The last time we were in San Juan, we just walked from the ship over to the fort and around town. I think it was $6/pp to get into the fort and the church, but could be wrong, it wasn't much. We then walked around town, purchased some rum and went back to the ship. In St. Thomas, I highly recommend taking the ferry over to St. Johns and going to Trunk Bay. It is beautiful and there's a self-guided snorkel trail about 50 yards from the beach. All the ammenities are there, too. The ferry is $6/pp, the cab fare to Red Hook to catch the ferry is $18/pp and the cab fare to Trunk Bay is $4.50/pp, can't remember the entrance fee to the park, but it's not much. Or you can stay on St. Thomas, Coki Beach and Sapphire Beach are the best for snorkeling. Coki has better ammenities, but Sapphire is nice, too. The cab fare is around 15-18/pp to get there. There was no charge to get on Sapphire when we went last and Coki I can't remember. We may have paid, but I know we had to pay to rent chairs. Last time in St. Maarten, we did the TikiHut snorkel, through the ship. It was great snorkeling for St. Maarten, which isn't the best place to snorkel. However, the operators of the excursion were fantastic, the facilities were great and the best part was there was no time limit. You could stay as long as you liked. They'd either drop you off for shopping or take you back to the ship. If you get dropped off for shopping, the water taxi is $6 or $10/pp, can't remember, but not much. The shopping in St. Maarten is just as good, if not better than in St. Thomas.
Have a wonderful cruise!