Agree with Wayne on Mrs Macs
It was a few years since we were there (2004) but here a couple of things to consider.
We stayed at the Howard Johnstone in Key Largo which was ok but the best thing was there was a natural spring outlet into the sea and Manatees would come by to drink the water. We saw and swam with them a couple of times. The first time one appreared right by me I almost had a heart attack, I thought it was a shark for a second. They are great creatures but it is illegal to touch them.
We hired a glass bottomed boat and skipper at Duck Key Marina and went out snorkelling. She also supplied SCUBA gear for me and SNUBA for the kids, its like snorkelling but you have a regulator on a hose connected back to the boat so you don't need to worry about breathing in water etc.
In Key West the Mel Fisher museum is great. Mel Fisher was a treasure hunter and he found a sunken Spanish Treasure Galleon, mainly silver, some gold and Emeralds. You can even pick up a gold bar to feel the weight of it.
If you are doing the Everglades, you might want to consider Shark Valley Visitor Center. They have cut a road out of the swamps which is a loop. You can either walk, hire bikes (best) or take a tram. We saw Alligators, Turtles, Snakes, fish, birds, insects.
Hope you have a great time.