We always stay at Pirates Cove on Marathon Key. They have a small beach, outside the season they aren't extortionate either. The further down the chain of Keys you go the more it will cost you. And you can drive to Key West from Marathon in an hour or so for a daytrip. I believe the season ends in late March but it might be in early April. And there is a short lobster catching period in their too where they may jack up the fees again.
If you go down there you should take one of the many snorkeling/scuba boat trips to the reefs. Not expensive, they will provide all the gear, and they put trained people in the water with you. Yopu will have a much better time in the Keys than you will on the Gulf coast.