Loucuras do Sul is a well-regarded restaurant in the historic centre serving a combination of Maranhão regional food and churrasco (Brazilian barbecue). The regional dishes are the real draw: torta de camarão (shrimp tart), vatapá, crab rice, and the ubiquitous arroz de cuxá. Portions are large and prices are reasonable.
The restaurant occupies a restored colonial building with azulejo-tiled walls and an inner courtyard. It's popular with local business lunchers as well as tourists, which is usually a good sign. In the evenings, it sometimes has live music — regional rhythms including bumba-meu-boi — though the schedule varies and it's worth checking ahead.