We were on this same cruise in 2010 & here is what we did:
St. Thomas - I have been to Magen's bay, done the St. Thomas Island tour as well as gone to St. John...definately book... St John trunk bay beach & snorkel trails. Beautiful beach & snorkeling.
Barbados - we just hired a driver, Emmerson Best, who charged us $30/pp & drove us everywhere, he stopped to get us rum punch, we got some food, went to a cigar plant, golf course, etc. He is the best! his email :email@example.com. Of course, all you need to do is get off the ship & the vans are waiting there for you. He will take you wherever you want to go. We have also been on the Jolly Roger Pirate ship in Barbados & had a great time.
St. Lucia - we did the land & sea excursion. They took you all over the island by bus, you got to see alot of the island (plantations, towns, etc. & then you spend the rest of the time on a catamaran & had lunch, snorkeled, saw the island from the sea. Overall, a wonderful way to experience the whole island.
St. Kitts - we just got off the boat & shopped. Be careful, there are guys holding monkeys which are very cute & they let you hold them, take your picture with your own camera & then charge you for doing so. Be careful about those charges.
St. Maarten - I have done a catamaran & snorkel through Carnival & then the last time, we just rented a taxi & went to Dawn Beach. We had great food, cheap drinks, played in the water, snorkelled & saw St. Bart's in the distance. It is a quiet but wonderful beach.
Enjoy the cruise....these are some of my favorite islands.