The ship will offer a sighteeing shore excursion that will drop you at the airport if that is of interest. We usually just grab a taxi to the Marriott Hotel on 17th, and either hang out there or take a long walk. There are plenty of restaurants on 17th Street, or you can grab a taxi at the Marriott and go to the beach (only a mile or two away) more restaurants and shopping. If you want to explore the INtercoastal, grab a pass on the water taxi for some interesting sightseeing. Lots to do in Ft. Lauderdale.
Hope this helps.