Heading back towards Praça XV  from Igreja do Mosteiro de São Bento , along the waterfront is the Espaço Cultural da Marinha (Av. Alfred Agache, Centro, near Praça XV, tel. 21/2233-9165, www.mar.mil.br/sdm/ecm/ecm.htm , noon–5 p.m. Tues.–Sun., free). Its collection of antique maps, navigating equipment, and buried treasure rescued from sunken ships in the Baía de Guanabara offers an engaging glimpse at Brazil’s maritime history. Part of the exhibits are housed in a floating submarine.
From here, you can take a guided boat trip across the bay to the Ilha Fiscal, the site of a neo-gothic castle that housed a customs collection center. The extravagant construction resembles something out of a fairy tale: It was here that the last legendary royal ball of the Brazilian empire was held—one week before the declaration of Brazil  as a republic.
Boat trips depart at 1, 2:30, and 4 p.m. Thursday–Sunday. Tours last 1.5 hours and cost R$8.