Hire a car, put the luggage in the trunk, go shopping and have a nice last lunch. far better than sitting at the airport for hours with a book...
There is a Left Luggage Section at Miami Airport but I have never used it so cannot say if it is ok or not. You say shop or tour are possibilities so why not rent a car at Fort Lauderdale and drop it off at the airport in Miami when you are done. This will give you so much more oportunities and solve the luggage problem at the same time. .... Neil
p.s. If you have a Sat-Nav with the USA on it take it or hire one with the car they are so useful there.
I have used left luggage at MIA twice now and found it very reasonable and secure. Get off the ship early and take a cab staright to the airport. Get the luggage put away and then you have a few options, the closest is a $25 - $30 cab to Dolphon Mall or try catching the shuttle bus that stops outside the main departures pick up drop off just outside of the covered bit and central. The times and prices are online if you search. This is a good mall if you are into shopping. We have also gone one better taking the everglades excursion that drops you at MIA about 11.30 then left luggage and a cab to Dolphon mall for a couple of hours where the food court offers a good choice for some late lunch and a shop. By the way we love the freedom of the seas,.
..for the cost of all those cab rides you could hire a car for the day.
We had a disaster trying to leave luggage at Miami, we queued for nearly an hour and before we got to the hatch the room was full and they closed storage. We now always book a room at the onsite hotel, we can store the bags in our room and have a lay down before the flight, there is a great restaurant in the hotel which looks over the airport. It is great to nip downstairs, check in the bags and then go back to the room for an hour or so. Sorfry cant remember the name of the hotel but sure you will find it on line. If not have a limo meet you from the ship and take you sitseeing for the day and drop you at the airport.