What ever you do, you have to go to Mallory Square around sunset, there are many performers there, from trained cats to a bagpiper, and even a man that lays on glass and has people walk on top of him.
If you like you can visit a shop that sells all the trinkets that the street vendors sell, but for way less.
You can visit Hemingway's home, and go to the 2 Sloppy Joes in Key West, the old one is down the street and around the corner from the one that is on Duval Street.
And if you think you could how your alcoholic beverage as well as any sailor, you could attempt to do the "DUVAL CRAWL" that is to have 1 drink in each and every bar on Duval Street. GOOD LUCK WITH THAT ONE.
The U S Navy has a base that use to be President Truman's Winter Home.
Truman annex, and that s where the true Southernmost point in the Continental U.S. is, but you cannot get there if you are a civilian, look it up on Google MAPS
https://www.google.com/maps/place//@24.5442051,-81.8038318,17z/data=!4m2!3m1!1s0x88d1b6dc4e0eb3eb:0xda5dd3859cd94bdd
Everything you see will say Southern Most, restaurant, cigar shop, T shirt shop, sandal shop, museum. A great museum is the one showing artifacts from various wrecks that have been discovered nearby.
You can go snorkeling for a 4 hour trip, the sail boat will take you to the edge of the reef, and in some places 4 miles from land, you can stand up and it will not be over your head, then 10 feet away, the end of the world is what you see, a bottomless dark hole.
There are many nighttime cruises for your enjoyment. You should be able to find a glass bottom boat cruise if that is something you like.
One thing you will find, the residents of Key West really invented the LAID BACK lifestyle.
I was in an open bar, having a cigar and a drink, around 9:00 p.m., when down the alley came an ambulance, it stopped at the bar, the guy ordered a 6 pack of corona, said he was going to a real bad accident.
That same night walking back to the Navy base, I saw a night watchman doing his rounds, drinking a Corona.
On one end of Mallory Square there is a restaurant, and a pier, that you can eat on the pier, it is covered with a tarp, and while you are on the pier, you get the feeling of being very slowly rocked, because it is a floating pier. While at the pier, a motor boat came along side, and blasted his horn, the waitress, went there to see what was up, he motioned to her, she lowered a bucket, and he put in it his request and some bills. She went to the bar tender, got his request 2 six packs of Corona, lowered it down to him with his change.
You can fish from shore if that is your thing. I believe you will need a license, but remember almost anything you catch from shore should not be eaten, because of reef poisoning.
You can visit Turtle Crawl, and they have a fantastic RAW bar. There is a large gay population, and a lot of artist as well all over the island. And you can visit Margarita Ville, so all in all who will have time for taking pictures?