L2C,
Been to Key West several times this year. Most everything is within walking distance if your into some good excercise. Most everything you will want to see is on Duval St...Margaritaville, Hog's Breath, Hard Rock, Harley Davidson etc. Mallory Square is also pretty neat. There is the Mel Fisher museum and lots of other interests. Southernmost Point is at the end of Duval on the right about a block or two over.
There are two piers. One is Mallory Square and the preferred place to dock and you can walk right off. If another ship is there then you might have to dock at the Naval Base. You are not allowed to walk through the base for obvious reasons. In this case free busses and train trolley's will pick you up and drop you in Mallory Square area. About 5-10 minutes at the most with an informative mini tour of the area with good info. Been both ways either is just fine. But you will have to meet back at Mallory Square before to catch your free ride back to the ship if your docked at the Naval Base.
Hope this helps!
CD