If you enjoy a stroll, walk up Duval street to Hemmingway's house, then visit that. Back to the dock area, turn right and explore the street that goes toward the private yacht docks (Wall Street). Some cool stores along there, and you'll quickly find the yacht basin. You might glimpse some pretty awesome crafts.
Back to the dock area, consider the pirate museum, but walk around and enjoy any street performers you might see and the colorful displays outside all the shops.
You might not be able to do all of this, but you will enjoy the sights.
OTOH, if you aren't the strolling type, right off the ship in the market area there are a couple of trolley companies to choose from. Either one is fine, really. They'll take you around the western end of the island. You probably don't have enough time to hop off, but you'll see a lot of great sights.