The most spectacular day trip from São Luís is to the Lençóis Maranhenses, a national park of white sand dunes and turquoise lagoons about 250 kilometres east of the city. The dunes are uninhabited desert-like, but the hollows between them fill with fresh rainwater from January to September, creating thousands of crystal-clear lagoons you can swim in. Getting there requires a bus or guided tour (most visitors go with a tour operator from São Luís), but it is one of the most otherworldly landscapes in Brazil.
Alcântara, a colonial city across the bay from São Luís, is accessible by fast ferry in about 90 minutes. Once a prosperous cotton-trading town, Alcântara is now atmospheric in its decay — ruined mansions, a tilting pillory column, and streets where grass grows between the cobbles. It's the melancholy counterpart to São Luís's bustling renovation.