Manaus has some very excellent 19th architecture
This theatre was build when the rubber prices soared up and there was a gigantic production in amazon.
Sadly the british stole our latex tree seeds and created a large scale plantation in asia,bankrupting the amazon economy for decades.
This has made the Brazilian government really paranoid about biopiracy.
I don't have a opinion in manaus,i never really visited nor made any research about it.
But you are going to probably visit the zoo,the old architecture mentioned above and probably going to set up a tour of the amazon jungle.
Just make sure to prepare for any eventual dengue or malaria fever,which are extremely common in the region.
also try to never use the SUS,or public health system,it sucks really bad even if you get privileged treatment because dying foreigners in hospitals are bad for international relations.
try to get a travel health insurance instead.