We went to Key West about 6 years ago. We liked it alot. But as someone stated in an earlier post, there isn't alot of beach there. I think there are a couple, and that's about it.
We went on a 1/2 day snorkeling tour, and really liked it alot. They also have 1/2 and full-day deep-sea fishing tours. They also have golfing.
Lots of nice/fun restaurants and bars, and lots of shops to browse through. We also thought it was fun going down to the pier and watching the cruise ships come in and depart.
They have lots of nice B&B's in Key West also. But we stayed at Best Western, just a couple miles outside of town. It had very spacious rooms, clean, screen-enclosed balconies, nice pool, continental breakfast, and a tiki hut at the pool that opened around 11am till around 6pm, for sandwiches, salads, soft drinks, and alcoholic drinks.
You can buy a bus pass, and use it to get back and forth from the hotel to downtown (or you can use a taxi). I think the bus basically just goes around the island all day long, and stops at various hotels for transportation to town.
We did that Dry Tortugas day trip, but we weren't all that impressed with it. There is one side of the island where you can go snorkeling though. I remember it being a VERY ROUGH boat ride out to the island. I think it took at least and hour to get out there, and because they go through a channel, the ride was extremely rough. Several people got sea-sick. We didn't, but we had each taken 1 Dramamine before going.