for a small as Key West is; there are a lot of things to see:
what interests you?
there is a shipwreck museum; the lighthouse is a must!; Hemingway's house; etc. I like to walk so I walk from the pier to what I want to see; some people like to get the trolley and then buy tickets to the museums, etc.
if you are into history; there is an old cemetary - long walk- but has interesting tombstones with some history.
Carnival Victory March 9, 2013
*carnival dream *
Glory 2/28/2010: Dina & Marty
Valor Feb 2009: Dina & Marty - again
Grandeur Feb 2008: DTW, Dina & Marty
Inspiration May 26, 2007
Fascination: Feb 2007
Enchantment of Seas: Sept 2006
Radiance of the Seas
Majesty of the Seas
The Big Red Boat
If you have never been there go ahead and take a tour. They drop you off in town afterwards and you can take any tram back to the ship. Take the early tour and then add something else you found interesting such as msblackjack suggested. Or if you can wait to get in the water take a dive or snorkel trip.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Alaska 2014 - haven't picked a cruise yet
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, Caribbean 2013
My wife and I booked the Conch Tour Train tour via their website prior to our cruise in October. The ship's excursion (RCL) didn't start till 11:00 but because we were able to breeze through imigration we were able to take the 9:30 tour. The tour lasts about 90 min. and gives you a comprehensive view of Key West and was well worth the $26. p/p price. We then walked down Whitehead Street to the Ernest Hemingway Home and Museum (again we were able to get discounted tickets from the Conch Tour Train) and continued on to the Southern most point. On our return we walked down Duval Street past Sloppy Joe's and had a piece of Key Lime pie by the Mel Fisher Maritime Heritage Museum.
Remember, "wear comfortable walking shoes" because you will be doing lot's of it and by the end of the day we were exhausted.