The South Travel Guide
Border Crossings at Foz do Iguaçu
U.S., Canadian, and British citizens need only a valid passport for stays of up to 90 days in Argentina, so crossing the border at Iguaçu isn't a problem. Crossing back into Brazil from Argentina, however, is a thorny issue. In theory, all U.S. citizens need a visa to enter Brazil, so make sure your paperwork is in order before you depart.
Many local taxis (both Argentine and Brazilian) have "arrangements" with border control and can also get you across with no visa. Most charge 150–200 pesos for the return trip. Though the practice is well-established (most hotels and travel agents in Puerto Iguazú have deals with Brazilian companies and can arrange a visa-less visit), it is illegal. Enforcement of the law is generally lax, but sudden crackdowns and on-the-spot fines of hundreds of dollars have been reported.