When I was young (I think I was 18 years old), me and my dad traveled across all the length of Amazon river (from Belém to Manaus) in a crowded boat sleeping on hammocks, it was a 6 days trip. https://goo.gl/maps/BFkCPx1HR1n

In some places the river was really narrow and almost could be crossed by swimming, but there where times that I couldn't see any of the two margins because of its enormous width.

Is was a really great experience, got to talk to indigenous people, saw a lot of botos cor-de-rosa (https://en.wikipedia.org/wiki/Amazon_river_dolphin) and got to eat amazing local dishes.