We did not book a tour. We walked around and did some shopping (you can get some good deals there)- finally the kids wanted to go back to the ship and we were happy to oblige. DH and I then went back to Front Street and had a blast. Highly recommend the frozen drinks (in which the selection is huge) at Flying Monkeys Saloon at Fogarty's. Dh and I agreed Key West is very nice and we would like to go back some time for more than a day.
Loved GC as well. We booked a tour for snorkeling and swimming with the Stingrays independently through Moby Dick Tours. Price was right and we had a great time. Snorkeled at the reef (saw a nice long eel- way cool) and great fish and then went to the sandbar to feed and hold the Stingrays. Make sure you hold the octopus food correctly because they have a super strong suction. Very cool as well.
We did the Canopy Adventure booked with Carnival. It is a little pricey but worth it. We all had a great time. It includes the Chairlift ride (as much as you want) then ziplining- several different lines and lengths. The guides are great and you feel safe and secure. One girl in our group was very scared and she even ended up loving it. Then you take the bobsled ride- I rode tandemn with DH so I didn't have to worry about braking and speed. He just let it rip and it was a blast. At he top they have a very nice pool and waterslide my son enjoyed. The rest of us forgot our swimsuits. :( There is also food and gift shop up at the top.