I think that there are a couple of better options. if you go to John Pennekamp park you could go snorkelling which ours really enjoyed but need to book in advance as very popular. also, just before the keys we went on a canoe safari through the everglades. brilliant day out.
Key west was ok but we found better things to do whilst we were there.
I think its a small key about a third of the way down. We are staying at Hawks Cay resort. we plan on swimming, snorkeling, eating, drinking and fishing. we are taking 2 teens so i think there will be lots to do.