Castillo de San Marcos National Monument

Go for the cannon firing. The rangers explain how the soft coquina walls swallowed cannonballs instead of shattering, and the whole reason St. Augustine still exists suddenly makes sense. Afterward, wander into the side streets off St. George — the main drag is touristy, but one block over it's courtyards and shade and much better than its reputation. Heads up that there's a long seawall project going on outside the fort into 2028, so expect some fencing. The fort itself is open and unaffected.

The 17th-century Spanish fort on the bayfront, built out of coquina, the local shell stone that absorbed cannonballs instead of shattering. You can walk the gun deck and look out over the water. Open daily, last entry at 5. They take cards only, no cash.