I have been to Key West several times so there was nothing new for me to explore. The thing to note with Carnival is that they tend to dock a lot at a naval base so one isn't able to walk off and enter into the downtown of Key West like I did on other cruises. For this, because it is a naval base, one has to take a little trolley into the downtown and back (coming back a US Naval officer will check your ID to make sure that you are indeed the person who should be going inside the protected area). This time around, I took the trolley, rented a bicycle and just enjoyed a nice long bike ride. 3/4 of the way through I found a McDonald's, had a coffee and utilized their free wireless to check email.
Being here several times as well, this trip I met up with others from the ship on the outside and we hired a taxi to take us into the downtown (total cost for each person around $3 each). As I am an avid bike rider and walker, I realized after getting there, that it was not that far at all so I explored the downtown by foot, popped into a few shops then walked back to the ship --roughly six kms. Grabbed a few nice pictures along the way and was still back in time for lunch with hours to spare for tanning. If you do this, just bring comfortable joggers and a big bottle of water and you will be all set.