I used to not reply to these because I thought it was a shame to just spend several hours in Key West, but...
As you get off the boat you can walk around Mallory Square, then look for a large red building on the other side. It is associated with El Meson de Pepe, and inside is a store with Key West and Cuban related things. You could look at these, watch the man make cigars, and maybe buy a momento. Then walk out the front and lokk at the market with postcards and various things to buy. But then find Whitehead. If you walk down Whitehead you will see the Audubon House and gardens, then Kelleys. If you walk farther, you will see, on your right, the lighthouse which you can enter (at a small cost) and see the house and grounds and climb up to the top. On your left is Hemingway's house which you might want to tour. (8.50 / person now). But then keep walking south to the Southern most point, take a photo (the 1,000,000,000th, in all likelyhood) then turn left on South Str. and watch for the Southernmost House. You can tour this as well, then walk north on Duval.
If you do all of this you will use up your time, but have enough left over to have a drink at The Hogg's Breath, Sloppy Joe's, Margueriteville, or Capt'n Tony's. Then a quick jag to the right on Greene then Caroline and over to the Key West Bight to see the ships and the bars. That sound you hear is the ship telling you to return, so hussle back.
Key West in a nutshell. Better to stay a week next time.