Just above Marathon Florida is a fantastic little beach park. Because it is a few miles north of the main action of the city center, there is not that much people traffic, so you have it almost to yourself. The water is outstanding and they have kayak trails you can use if you rent a cheap kayak. There was about eight different kayak boats out, and just a handful of people swimming or laying around the beach. Great place.
When you look down on the area using the earth mapping tools website, it is such an impressive area. The kayak trails are a must do for the Florida Keys. This mangrove trail system is easy to find and much easier than what is up around Robbie's Marina in Islamorada. Most people just fly past this Beach Park at 60mph and would not even know how nice it is.
Please add my comment regarding the Valhalla Beach Resort, which is a super place to stay and right next to Curry Hammock State Park. Yes, come join us and take advantage of reef diving, fishing and Kayaking in true Florida Keys style. Take a charter boat and head out to the Deep Water fishing grounds.
Key Colony Beach is just up the street, a great little island to rent a weekly condo. You can kayak in Vaca Cut and in the canal system. Very nice location for a vacation adventure. Bring Your Boat or rent one.