Fort St. Catherine is a well-preserved military fort on the far northeastern tip of Bermuda, about a 20-minute walk from quaint St. George's. It was established in 1612 to help protect Bermuda against Spanish attack, and remained in use for hundreds of years, being rebuilt a number of times over the centuries.
Today, the fort is set up as a history museum, with a decidedly military bent. You can wander the tunnels, supply rooms, and ramparts, all peppered with cannons, dioramas, and exhibitions on such topics as the importance of the fort following the American War of Independence, when Bermuda became essentially a British naval base protecting the western Atlantic trade routes.
It's an informative, if sometimes dry, visit, unless you happen to geek out on artillery through the ages. If you don't feel the need to read every single inscription, you could be out of there in an hour and on your way to nearby Tobacco Bay.
Entrance is $7, and the fort is open 10:00 a.m. to 4:00 p.m. daily. There's a passable restaurant next door if you need refreshments.