According to the U.S. Department of State on Foreign Entry Requirements for U.S. Citizens:

Argentina [passport, no VISA for up to 90 day stay]
Argentina Visas: Nationals of the USA, Canada, most Western European countries, Australia and New Zealand do not need visas to visit Argentina. In theory, upon arrival all non-visa visitors must obtain a free tourist card, good for 90 days and renewable for 90 more. In practice, immigration officials issue these only at major border crossings, such as airports and on the ferries and hydrofoils between Buenos Aires and Uruguay. Although you should not toss your card away, losing it is no major catastrophe; at most exit points, immigration officials will provide an immediate replacement for free.
(Source: www.lonelyplanet.com)

Chile [passport, no VISA for up to 90 day stay; $100 entry fee will be charged at airport]
Chile Visas: Citizens of the USA, Canada, Australia, New Zealand and most Western European countries do not require a visa although US citizens do pay a 100.00 levy (cash only). Canadians pay a 55.00 levy and Australians 34.00, while there is no levy for EU or New Zealand residents. A 90-day entry permit, renewable for another 90 days, is received on entering the country. (Source: www.lonelyplanet.com)

Costa Rica [passport and return ticket required but no VISA]