The best "secret excursion" I've had was to go in with 2 other couples and rent a power boat and captain from nautinymph in St. Thomas. We had a glorious day touring the islands for about $75/person.
The second best was renting a car on Guadeloupe to tour Mt. Soufriere (volcano), the country side, and the waterfalls. It was a VERY busy day, and just about missed the ship :-), but what fun!
I really don't know if there are many "secrets" left on the Carib. Anything that has market appeal probably has an excursion to it. Those that don't don't have wide appeal, are dangerous, illegal, or there's not enough time to complete it.