I was in São Luís, capital of the State of Maranhão, for a weekend last August. Most tourists don't spend much time in São Luís, rather go as soon as possible to the nearby Lençóis Maranhenses, a paradisiac complex of fresh water lagoons formed on the dunes.
Anyway, São Luís was the only city in Brazil founded by the French (hence named after the French King St.Louis IX), but very little of French heritage is left.
The main attractions are the Government Palace, some historic museums and the "tiled buildings", designed with the purpose of isolating their interior from the intense heat (the city is close to the Equator).
From all the 16 WHS I already visited in Brazil, this was the one I least wanted to go - it is far from São Paulo (a 3,5 hour-flight)and most buildings lack proper maintainance -, but I ended up enjoying. In any case, it is usually interesting to go to the least visited and off-the-beaten-track WHS.