Fort Zachary Taylor is a mid-19th-century masonry fort at the island's western tip, now part of a state park that includes Key West's best beach. The fort itself is substantial and partly preserved; guided tours run daily and explain its Civil War history. The beach attached to the park is where most locals swim — cleaner water than Smathers, with Australian pines providing shade and decent snorkeling directly off shore.
The park is a short bike ride or taxi from Old Town. Entry costs a few dollars. The combination of a genuine historic fortification and a good beach makes this one of the better afternoon options on the island, and the walk to the water's edge gives a clear sense of how narrow the passage to the Gulf is at this point.