I know it sounds expensive, but you might consider renting a car. That way you don't have to worry about lugging you luggage around with you. Since 9/11, you have to keep it with you. There are some nice places to visit in Miami, Villa Vizcya, the Seaquarium, the Crandon Park Zoo, South Beach,Miami Museum of Science, etc. It beats hanging around the Miami airport for 8 hours, of course, beating your head against the wall would be better
. Have a wonderful cruise!