Cruise ships can only dock in two places at Key West - downtown or at the Navy base. Downtown is better, but even if you dock at the base the cruise line will provide complimentary transportation to and from town via the Conch Tour train - which is really some open air cars pulled by a modified truck. It is quite pleasant. If you dock downtown, you will be right in the middle of things as soon as you leave the ship.
We went to the Belize Zoo. It is interesting but the ride is long and there isn't much in the way of refreshments at the zoo. When we were there, the restrooms were outhouses....really rustic.
You say you were ripped off at Cozumel, but your comments indicate you did a mainland tour to Tulum. Is that the case? I say this because Cozumel itself is a great place and I have never been pressured there.
I would avoid the Cozumel highlights tour and just go out on my own. It is quite safe and you'll see more on your own.
Xcaret is fine, but it eats up lots of time and with the exception of the cave tubing you can duplicate most things done at Xcaret on the island of Cozumel.