We've been to Key West many times. There's lots to do there. I recommend that you go on the trolley. You can get on and off at the various stops. It's a little pricey but the ticket is good for the entire day. We enjoyed the Hemingway House, Southernmost Point and Mel Fisher's Shipwreck Museum. I'd take a peak into Sloppy Joe's just to say you've been there. Of course, it's going to be hot in Key West in September so you might want to stop for a beer instead of just a look.
You're right, we did live in Austin. In fact, we moved to Florida from Dallas so we know heat. The humidity down here will is what kills you.
You might want to check the website www.keywestfun.org. Earlier in the year they had 2 for 1 coupons for some of the Key West attractions like the Hemingway House, Mel Fisher's Maritime Museum, Truman Little White House and the aquarium. The coupons expire on Oct. 15. Sorry, no coupons for 2 for 1 beers at Sloppy Joe's.
We went to Blond Giraffe-great Key Lime Pie. Several famous bars, so you can arange a bar crawl, to learn the history and taste whatever drink they are famous for. Stock Island is about a 20 minute ride from the port, Stock Island has a reality show taped there for MTV. Parasailing, lots of gift shops, etc. We did not tour the Little White House, but walked by and heard from others it is intersting and worth taking the time to see it. It is still used by the presidents for a place to stay when in Key West.
As a resident of South Florida, I have visited the Keys numerous times, and I agree with doing the trolley tour. It stops at all the places folks need to see, plus keeps you off your feet walking. The main street in Key West is Duval Street. That is where the action is with tourists and locals. So even if you decide to not take the trolley, Duval Street is where you want to be.
'Penny 3333' is absolutely right about the key lime pie...Blonde Giraffe is the best. More places on your 'have to stop in tour' are Sloppy Joes and Hogs Breath Saloon. The conch fritters at Sloopy's are the best I have ever had. Hands down. Both places sell t-shirts (little pricey) but worth it to say you've been there. See if there is a place called 'Pappy's' which is near Sloppy Joes. They have the most amazing fried shrimp....oh Lord!!!
One thing about the Keys: they are laid back. Its not about five minutes ago down there. People are very friendly and willing to help. I will not say there are no interesting people down there, but man, are they fun to watch. When you get off the ship, look for a local newspaper. It highlights all local hotspots that tourists don't know about. Hopefully, your ship does not leave until evening. The locals have a celebration at Mallory Aquare every night as the sun sets. Awesome!!
We were in KW last Thanksgiving. There are lots of fun things to do. Here are some of the most easily accessible ones.
Duval St. - The main drag through KW, where most bars and shops are located.
There's a webcam outside of Sloppy Joes where your friends at home can see you. We called while on camera and it was kind of fun.
The top floor of Hotel La Concha has a bar with an open air deck that has a great view.
Lighthouse - you can climb to the top and get a nice view of the island too, but no drinks.
Hemingway House - kinda neat, especially the 6 toed cats.
Audabon House - not that great, esp. since it wasn't his house. He just stayed there for a week.
Mel Fisher Museum - pretty interesting, esp. if you're intrigued by the whole treasure thing.
Cemetary - sounds creepy but there are some interesting headstones like the one that reads, "I told you I was sick!"
Rent a bike or moped - good way to see a lot fo the backstreets with interesting houses and flora, and you can get to the public beaches on the other side of the island.
There are a ton of good restaurants but they tend to be pricey.