Love Key West!! Been there many times. We took a 4 day cruise on Majesty of the seas and KW was one of their stops. All the posters above named the best places to visit. Duval Street is the most popular to shop, have a drink, etc etc. You can also visit Ernest Hemmingway's house, stop by Sloppy Joe's for a cold beverage, go to Fort Zachery Taylor for some beach time. They have a small diner on the beach to take care of the hungry tummy, go to Southernmost Point for the obligatory tourist photos and just enjoy.
They also have the Conch Train Tour where for an hour they will take you all around KW and describe all the attractions. You can even get a bus pass and map of all the main stops and do a mini self guided tour yourself. You can get on and off the bus all day as you go around the town.
Drive time is all of 8 hours. 5 hours to Miami and then approx. 3 more hours to Key West.
Have a Blast