Most 3 day cruises out of South Florida are Bahamas cruises. You likely leave on a Friday and return on a Monday, and visit Nassau, Bahamas and a private island in the Bahamas owned by the cruise line you are sailing on. While you are in Nassau you can visit Paradise Island. Popular shore excursions while in Nassau include Nassau town tours, take a trip to the Atlantis resort in Paradise Island, take a sail boat & snorkel or swim with the dolphins. Many guests forgo purchasing a shore excursion and just walk from the ship into town and visit the straw market and local restaurants and bars.
Out of the Port of Palm Beach, you can sail on a 2 day cruise which visit Freeport, Bahamas. In Freeport you can also swim with the dolphins, take a grand Bahama Island waterway tour, visit Lucayan National Park and tour the caves or just spend time enjoying the beach.