My mother and I visited Key West on a spur of the moment. She came to visit me for a few days, expecting warm weather in southwest Georgia where I live. Unfortunately, temperatures were lower than usual. So we decided to go to Key West -- it's known for its year-round tropical breezes. Both of us wanted to visit Key West one day day anyway so we made reservations and packed in about 15 minutes. :)
Key West is a charming town with gorgeous Victorian buildings, lots of shopping opportunities, good places to eat, plenty of attractions, etc. It's considered to be the southernmost tip of the United States, even though Dry Tortugas is really the last island -- it's about 70 miles from Key West, not connected to the mainland; however, you can take a one-day trip there by boat. Key West is the last key connected to the mainland. It's located about 90 miles from Cuba.
Although I recommend driving to Key West (maybe from Miami?), you can also fly there. I enjoyed driving through the keys because the view is gorgeous (Atlantic on one side and Gulf of Mexico on the other), there several towns where you can stop, Theater of the Seas is definitely a great place (see my comments), etc.
Our hotel was located only a couple blocks from Duval Street, main shopping and attractions area. So we walked everywhere. If you don't want to walk, you can rent a moped or electric car or simply ride a bicycle.