Castle Clinton National Monument, Battery Park, NYCThis sandstone battery built to defend the city’s harbor during the War of 1812 was fully armed and yet it never fired a single shot except for target practice (it was also off-shore until a landfill expanded Battery Park). Since then it’s hosted opera shows, newly arrived immigrants before Ellis Island opened, and an aquarium. Today it's a free little museum where you can buy tickets to the Statue of Liberty and Ellis Island.