Gwen, there is a lot of discussion on this board about the newly refurbished Royal Caribbean ship, Enchantment of the Seas. She does 4 and 5 day cruises out of Ft. Lauderdale to the western Caribbean - Key West and Cozumel on the 4 day itinerary, with the addition of Belize City on the 5 night.
It's a wonderful ship with lots of amenities, and some innovative features since the refurbishment.
Since your question is posted on the Royal Caribban board, I'll mention that Royal Caribbean has two other ships doing short cruises from Florida. The Sovereign of the Seas sails 3 and 4 day cruises from Port Canaveral (about an hour from Orlando), and the Majesty of the Seas sails 3 and 4 night cruises from Miami.
My vote is for the Enchantment of the Seas.