I would e-mail Carolyn at Barrett Adventures(sorry, I don't have their e-mail with me) in Jamaica. They'll e-mail you a list of the more common excursions they have for half and full days. Custom tours are also available. If you seach the message boards you will not find anything but very positive reviews for them. They have a great reputation and are known for doing what you want while offering local "out of the way" tours different from the mass produced type tours.
We'll be doing VS falls in the morning, lunch and tubing on the river afterwards with a stop at the Hummingbird sanctuary w/ shopping on our way back to the ship on January 20th. Best cruising to you.