Casa do Maranhão is a museum in the old customs building at the port, dedicated to the Bumba Meu Boi festival tradition. The building itself is a beautiful example of colonial-era port architecture, and the conversion into a festival museum has been done with care. The collection focuses on costumes, masks, and instruments from the various Bumba Meu Boi "groups" (the different performing organizations that maintain their own distinct traditions).
The museum has the most spectacular collection of festival objects in the city, and the scale of the costumes — some of them house-sized constructions of wire, fabric, and mirrors — is genuinely astonishing. Allow two hours if you're interested in the subject; an hour if you just want the visual impact.