St.Lucia = Sail along the coast to the Pitons. Fabulous trip with a snorkelling stop, visit a little bay where Dr. Doolittle was filmed, etc. The highlight though is sailing upto the pitons.
On Barbados, I loved Welchman's Gully (because of the many wild monkeys), Harrison's Cave, and the Botanical Gardens (don't recall the exact name). Barbados is a great place to take an extended island tour. The Atlantic coastline is scenic.
St Thomas: take a trip to Drake's seat to overlook Magen's Bay. One of the prettiest beaches in the world. Any of the beaches on St. Thomas are great, but I like Coki, Sapphire, and Secret Harbor for snorkelling.
On St. John, it's either beach/snorkeling or nature hiking/exploring