Kids will love exploring the many rooms and checking out the cannons atop the Castillo de San Marcos in St. Augustine. The oldest masonry fort in the United States, the Castillo de San Marcos stands between Matanzas Bay and the heart of St. Augustine. Visitors are allowed to roam the entire fort, so take extra caution when climbing to the top level. The walls are not very high, especially near the cannons, so keep an eye out when kids are in these areas. As part of the National Park Service, ticket prices are very reasonable, especially considering they are good for seven consecutive days. Plus, children ages 15 and younger are admitted for free with a paying adult.