This park, with its long, white-sand (and at times seaweed-strewn) beach, named Sandspur Beach by locals, is the big attraction in these parts. As Keys beaches go, this one is probably the best natural stretch of sand in the island chain. There's also the novel experience of walking on the old Bahia Honda Rail Bridge, which offers nice views of the surrounding islands. Heading out on kayaking adventures (from $12/36 per hour/half day) is another great way to spend a sun-drenched afternoon.
You can also check out the nature trails and science center, where helpful park employees can assist you to identify stone crabs, fireworms, horseshoe crabs and comb jellies. The park concession offers daily 1½-hour snorkeling trips at 9:30am and 1:30pm (adult/child $30/25). Reserve ahead in high season.