A national park right on the main Amazon River...
Amacayacu National Park in in the Colombian Amazon. I got a flight from the capital of Colombia, Bogota, and down to Leticia. The latter is Colombia's southernmost town and a major port on the Amazon river. It is the area where Colombia, Brazil, and Peru converge.
Because my Spanish is minimal, it was hard for me to arrange transport to the park, so I first went on a day trip up the Amazon. Here we watched hundreds of capuchin monkeys on a small island, watched 'Pink Amazon River dolphins' or 'Botus' (Inia geoffrensis) and swam in the river. On the tour I visited the park, and then arranged transport there the next day. I stayed at the park headquarters for a few nights. (My strongest memory here is of losing my binoculars. )
At the time they had an orphaned 'Woolly Spider Monkey' who latched onto whoever he could, and wouldn't let go. I got stuck with him, but still wanted to go for a walk in the rainforest. So I just walked into the jungle with this monkey on my back. But he was actually scared of the jungle, and only occasionally stretched out and picked at a plant.
There was also a hide here a little walk from the centre. I spotted a species of Tamarin (a tiny primate) and several of the Amazonas parrots, many of which I couldn't identify exactly because of my binocoular-less status. Sitting by the river I also glimpsed the 'Blue Amazon River Dolphins' or 'tucuxi' (Sotalia fluviatilis) rather more attractive than the Botu, and looking like a smaller bottlenose. I hired a canoe to paddle myself on the smaller blackwater back rivers, although I didn't see much this way. I also spot-lighted at night by myself and was treated to a tree snake and a Four-eyed Possum.
I must have been there in the drier season, for in the wet much of the area gets flooded and the ecosystem changes dramatically seasonally. In fact, it was reported that in 2012 massive floods closed the centre down: so check to see if it has re-opened.