I just realized that my ship arrives back in Miami at 8:00am and my flight is not until 8:00pm. I have found plenty of suggestions on what to do, but what do I do with my luggage and stuff for 12 hours.
Check with the cruiseline and see what excursions they have for Miami, this way they will take care of your luggage, and some will get you either back to the port or take you to the airport.
You can rent a car as well. We did in Ft. Lauderdale because we didn't want to lug our luggage around with us. The airlines wont let you check your luggage in until 4 hours before departure (at least that is what we were told). We through our bags in the car, and bombed around town.