Hi sq, congratulations on your up-coming Costa cruise! I hope you like them as much as we do, in fact, we love them. That's amore!
If you'll look down the main page, just a little further down, there's a Caribbean Port Chat section. Use your "Find" in the upper left hand corner and type in your ports, you'll find all the posts made about them. In St. Thomas we usually grab the ferry and go to St. Johns and then to Trunk Bay. The last time we were in San Juan, we just did a walking tour of the fort on our own, walked around Old San Juan, bought some rum and went back to the ship. It was a beautiful day, nice breeze blowing so it wasn't too hot walking around.
Best wishes, don't forget to try the gelato and pizza, fantastic!