There are a number of beaches in the Florida Keys, though maybe not as many as you would expect on a chain of islands. The best beaches in the Lower Keys are Bahia Honda north of Big Pine Key, and Smathers and Fort Zachary Taylor in Key West.
Bahia Honda is not just a beach, but a state park, and charges admission. On top of two different beach areas, this 500 acre park includes a nature trail perfect for birding and learning about different local plants, kayak rentals, and a gift shop and concession stand. There are camp sites and a couple of cabins for camping in the park.
Fort Zachary Taylor State Park is similar in that it is a state park that charges admission and includes a beach. It also includes a Civil War fort that runs tours, and concession stand and gift shop. The beach can be rocky though, so swim shoes are suggested. There are a large number of tables available, though you should still try to snag them early. However, due to a couple of rocky outcroppings just off the beach, it has some of the best snorkeling accessible from shore. Some evenings you can also even see a pod of dolphins while watching the sunset from the shore.
Smather's beach is a popular free beach in Key West. A long, gently sloping beach looks out to the Atlantic Ocean. Along the beach, there are kayak, kite board, umbrella and chair rentals, and a number of food and drink trucks along the road. There is also a public restroom, though it is at the westernmost end of the beach and can be a walk if you set up camp further east. While there are a couple of tables, they are generally claimed early in the day, and are further up from the beach.
If you are looking for nature trails and walks the National Key Deer Refuge, ranging from Big Pine Key to Sugarloaf Key, offers nearly 20 miles of it. Stop by the Visitor Center in the shopping center off Key Deer Boulevard to grab a map and find out the best places to spot Key Deer, the endangered and only deer species found this far south. Of particular interest is the Blue Hole, a quarry that has been filled with rainwater through the years, home to many species of turtle, bird, and the occasional American alligator. Key Deer will also come by to drink from it.
The Key West Tropical Forest and Botanical Garden also includes a small nature walk, available for a donation. With a variety of tropical plants and flowers to learn about, it is also a great place for birding and butterfly watching, though no catching. For a more up close experience with butterflies, there is also the Butterfly Conservatory on Duval street in Key West.
A list of all of the parks, facilities and boat ramps in the county can be found here www.monroecounty-fl.gov/facilities.