Crossing into Brazil from Argentina can be a thorny issue. By law, all U.S. citizens need a visa to enter Brazil. Visas can sometimes be issued in as little as three hours from the Brazilian consulate in Puerto Iguazú (as opposed to the three days they take in Buenos Aires) but can take up to two days. So it’s best to secure your visa, which costs $160, well before you expect to cross. The Buenos Aires consulate also has a reputation for refusing visas to travelers who don't have onward tickets from Brazil.
If you stay in Foz do Iguaçu, travel on to other Brazilian cities, or do a day trip to Brazil by public bus or through an Argentine company, you'll need a visa. There have been reports of getting around this by using a Brazilian travel agent or by using local taxis (both Argentine and Brazilian) that have "arrangements" with border control. 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.