You have to be in the designated areas by about 8:00 I think (since we didnít do this I am trying to remember). You would probably be off the ship by 10:30 at the latest. They do have port excursions that you can do if you have a long wait before your flight leaves.
I am sure you could take a cab to the beach or shopping if you needed to waste some time. The Miami airport is pretty large, so it has quite a bit to do if you just go right there after leaving the ship.
They also have an option if you fly with one of the larger carriers to check your baggage right from the ship. This would allow you to not have to transport it yourself and let you travel around the city without it. I think our friend said it cost $20 to do this.
I hope this helps and doesnít cause you more confusion. We flew out at 10:30, but then had a 3 hour layover in Atlanta, so we ended up hanging out in airports most of the day. Maybe next time we will drive.