Cape Henlopen is where Delaware Bay opens into the Atlantic, and it is the best stretch of public coast in the state. The park covers about 5,000 acres of dunes, pine forest, salt marsh, and beach, with a paved bike trail running its length and several swimming beaches with lifeguards in summer. The Seaside Nature Center near the main entrance has touch tanks, a small aquarium, and rangers who will tell you where the bird hides and the horseshoe crab spawning grounds are.
The military history is unmissable: Fort Miles, the coastal artillery installation that defended the bay during the Second World War, has been partly restored and is open as a museum, and the eleven concrete observation towers along the dunes are some of the most photographed structures on the East Coast. Climb the one at the park entrance for the view down the cape.