There is pleasingly little to say about visa requirements in South America, as the majority of travellers simply won't need one if you're just visiting.
Brazil and Peru are a little stricter than others, but if you're coming from anywhere in the EU or North America, you won't have a problem. If you hail from elsewhere in the world you should check ahead of time if you'll need to acquire a visa.
Citizens of Australia, Canada, and the USA will have to pay a fee of around USD100 to enter Argentina. This can be paid upon arrival.