One thing to add. IF you want a car after the cruise, you can have the rental car agency pick u up at the port. They'll bring you the the agency. YOu can then have a car for the day(if you have a late flight), take it back to the airport when you're ready to fly out.
We did this before. Rented a car at the airport. Went to dinner, a little shopping and went to the beach. They picked us up and took us back to the agency. We then did Kennedy Space Center for the day. Then headed back, returned the car at the airport and caught our flight home. Didn't have to sit around the airport for hours. This is if you have a late flight. Our always leaves about 6:30 pm.