Praça da Sé (Catedral Metropolitana) by glabah
The church is an impressive sight, and the various choas in the plaza (protests, venders, etc.) are interesting for people watchers.Be realistic, however. This building was completed in 1954 (after 42 years of construction). It is impressive, but it is the meotropolitan cathedral of the largest city in South America. It should be impressive.
The Brazilian guide book I have (Guia Quatro Rodas) rates the Basilica de São Bento (1922) as a much more interesting place for those who like visiting historic churches. The Ordem Terceira do Carmo (1632) is a much older building and is supposed to have wonderful details throughout the church.
Since there are so many that have visited the Metropolitan Cathedral (also called Catedral da Sé) I would suggest one of these lesser known churches might be more interesting.