nothing special exept the opera house, but a great place to be, a realy good place is to sit at the market drink a beer listen to the forro an watch the rio negro. Of course a startpoint to the rainforrest.
Manaus is a major hub being on Amazon river, at the heart of the jungle. I spent 3 days / 4 nights total here, 1 night trip up the Rio Negro on a tour to stay on floating barge hotel.
If you are looking for an Epic Adventure. Traveling in amazon river is quite an experience. Especially seeing the meetings of the waters
Here you are in the middle of the Amazon rain forest and you are in a city which is large and vibrant and on the Amazon river - wow. Busy and hot and humid with the jungle not far away.