Fort

San Cristóbal is bigger than El Morro and less visited, which makes it the better of the two if your children want space to roam. It covers twenty-seven acres of layered defences built to protect San Juan from land attack, connected by tunnels that are open to walk. The detail that lands with most visitors is in a small dungeon room, where someone imprisoned here centuries ago drew ships on the wall in charcoal - they are still there. Rangers give talks through the day. The same ticket covers El Morro, so most families do both, though not usually on the same day with young kids.

El Morro guards the entrance to San Juan Bay and has done since the sixteenth century. Six levels of ramps, tunnels, gun positions and the round garitas that appear on every Puerto Rico souvenir make it genuinely fun to explore rather than merely educational - children treat it as a castle, which it is. The enormous green esplanade in front of it is where San Juan families come to fly kites on windy afternoons, and vendors sell them at the gate. Your ticket also covers Castillo San Cristóbal a mile away, the larger of the two. Shade is limited on the upper levels.

Fort Christiansvaern is the bright yellow Danish fort on the Christiansted waterfront, and it is small enough to see properly with children in under an hour - ramparts, cannon, the powder magazine, and the punishment cells below. The surrounding historic site includes the old Danish customs house, scale house and West India Company warehouse, where enslaved people were sold, and the interpretation does not avoid that. Around it Christiansted itself is compact and walkable with arcaded streets and a boardwalk. Boats to Buck Island leave from the harbour here, which makes it easy to combine.

Getting to the Dry Tortugas is most of the experience - two and a quarter hours by ferry across open water, or a seaplane if the budget allows. What you arrive at is Fort Jefferson, an enormous unfinished nineteenth-century fort covering almost an entire small island, surrounded by water clear enough to see the coral heads from the walls. Snorkelling straight off the beach is the best in the state because there is no run-off and almost no boat traffic. Bring everything you need; there is no shop, and the ferry leaves on schedule with or without you. Rough seas make the crossing genuinely unpleasant, so watch the forecast.

Fort Clinch is one of the best preserved nineteenth-century masonry forts in the country, and unusually you can walk almost all of it - the ramparts, the galleries, the kitchen, the bunk rooms. Rangers in period uniform staff it daily and stay in character, which children find either fascinating or unnerving. The beach below the fort is a well-known spot for finding fossilised shark teeth, especially at low tide near the jetty. Two campgrounds sit in the park, one on the Atlantic and one under a canopy of live oak and Spanish moss. The road in through the maritime forest is worth the drive on its own.

Fort De Soto regularly turns up on best-beach-in-America lists, but the reason it belongs on a family list is North Beach’s lagoon - a shallow protected pool separated from the Gulf by a sandbar, where the water is warm, still and waist-deep a long way out. There is a genuine fort to climb over with its original mortars, seven miles of paved trail, a two-mile canoe trail through mangroves, and a large dog park with its own dog beach. The campground sits on the water. It is a county park, so entry is cheap.

Key West is short on good beaches and this is the one locals use. The water drops off quickly compared to Bahia Honda, which makes it less suited to toddlers but much better for snorkelling - the rocky edges hold parrotfish, sergeant majors and the occasional small barracuda within a short swim of the sand. Behind the beach sits the Civil War fort itself, with the largest collection of seacoast cannon in the country and cool brick casemates that make a good hour out of the sun. Water shoes matter here; the entry is rocky.