It sounds like you are on a cruise! If so, or not, for one day I would:
Walk down Duval street and stop at Hog's Breath Inn, then smile at the camera, and call someone you love and wave to them over their web-camera. Pick out someone who you know is in a cold place! Photo#1
Then start walking down Duval, and laugh at every crazy t-shirt in all of the windows, as you go to Jimmy Buffet's. Order a Cheeseburger in Paradise (It is really great) and a top shelf margarita. Picture 2
Keep going, and go to a neat shop called Cuba, and look around. Then keep walking, and get a picture of yourself at "The Southern most point in the US"!
Now, walk back to the lighthouse,(picture 3) take a tour, and go across the street, to Hemingway's house. Then finally, I would see Harry Truman's place before hitting another neat bar or two! You will see many!
Very important, make it back to Mallory Square for SUNSET! (Picture 4)
Depends on your interests. Although it's a small key there is alot to do, one day won't do it justice. If you are there to see the tourists stuff do what mosts first timers do and hop the Conch Train that tours the Key and points out the most interesting sites. Duval street is the main strip with the tourust bars and shops, also visit Hemingwyas house, the Southernmost point etc etc. If you enjoy a good drink in the tourist section Sloppy Joes, Hogs Breath and Captain Tonys are favorites, also try Irish Kevins for all day and night music. Off the beaten track for dinner try Kelly's (owned or at least was by Kelly McGillis actress from Top Gun and The Witness), also Blue Heaven, great places. Personally I prefer down by the Marina area, Conch Republic bar and restaurant and Schooners bar (go at night and see the local entertainment). Sunsets are great in Key West, most go to Mallory Square, I prefer the Top of the Crowne Plaza hotel on Duval Street, great views of the sunset and the entire key. Enjoy.