Manaus in the last city on the Amazon that most cruise ships travel to from the west. It is a big city that is the hub for this area. It is also where rivers meet and one of the most interesting things to see is what they call the "meeting of the waters." The Amazon muddy color meets a blue river and the two have different PH factors and they have a real process of the mixing of the rivers. It is so visual and dynamic.
Manaus also has a beautiful Opera House from the rubber baron days that should not be missed- You can easily walk to it and tour it without the shore excursion.
It also had some great tours back into the rainforest on small boats where you truly saw the rainforest way of life, many birds and giant water lilies.