This place is simply massive. Built on the eve of World War I, Copacabana Fort boasts walls of reinforced concrete 12m (39 ft.) thick, and a monstrous 305mm (1 ft.) cannon that could fire a deadly shell 23km (14 miles) out to sea. Inside, rooms contain then-state-of-the-art instruments for targeting and aiming the great guns. Best of all, the bored soldiers guarding the place never leave the gate, so you're free to touch, fiddle, and play as much as you want. Twirl the knobs on the great cannon until its muzzle points toward your hotel, trundle a shell over from the magazine and let fly. Allow an hour.
Skip the Army History Museum, also on the fort grounds, an amateurish and laughably biased tribute to the role of the army in Brazilian history. Coup? What coup?
Take a Break -- The Café do Forte, on the seaside ramparts of the Forte de Copacabana, offers some of the best views in town. Operated by the Confeitaria Colombo, the cafe offers outstanding pastries, cakes, and sandwiches. Open Tuesday to Sunday 10am to 8pm (tel. 021/3201-4049). To enter you must pay the R$4 fort entrance fee.