These wetlands in the South West of the country, in the states of Mato Grosso and Mato Grosso do Sul, is one of the the best places in the country for seeing wildlife. You'll have to take a tour for a few days or stay at one of the Fazendas (farms) who organize trips in the countryside.
Of course, as with all wildlife viewing, you'll need some luck but with a decent local guide you have a good chance of seeing capybara, tapir, anteater, armadillo and anaconda. For sure you’ll see cayman and many parrots and you can go horseback riding and pirana fishing. The Pantanal also has many jaguars but don't count on seeing those.
The Pantanal is not just a tourist area but also a cattle breeding area so you'll probably bump into some real cowboys.