We spent most of our time in Islamorada fishing, but just south of there you can feed the tarpon. You buy a bucket of bait and can either throw it out or let em come up and get it
A 150 lb fish eating out of your hand is pretty cool. The 'keys' are not too far apart but the speed limit is very slow so it takes forever to travel around. You are not too far from 'alligator alley' in the Everglades, I know you have seen tons of alligators but its a cool drive. Depending how old the kids are, lots of snorkeling to do and lobster season maybe in at that time. Be forewarned that the community of Key West is not much unlike the community around Bourbon Street, so just a heads up