If you have a month, catch a flight to see Iguazu Falls. It's better than what the guide books show. Stay on the Brazilian side; you'll need a visa to get into Brazil.
Go to the US Embassy and they will expedite the visa process for you. The Brazilian Embassy closes at 4:00 PM SHARP! We got there at 3:59 and they were shutting the door and they had our passports and it was Friday...
Rent the mid 80's movie "The Mission". It was filmed at Iguazu Falls.