Manaus is not a pretty city. The only nice attraction is the Theatre Amazon. Most people only pass through Manaus on their way to the Amazon with good reason.
The muncipal market is an interesting place but pretty much like any other big market in Brazil. It is also interesting to book a boat ride to see the mixing of the waters. There is not much to do after seeing it but it is one of the few places in the world where you see this type of thing.