Puerto Iguazu is located in the north of Argentina and must not be missed on any trip to Argentina. The city is located in the country triangle and has borders to Brazil and Paraguay.
The highlight are the Iguazu Falls (Spanish: Cataratas) in the Puerto Iguazu National Park. They were selected in a global survey to be one of the 7 new natural wonders. They are the most visited place in Argentina with over 1 million visitors annually. The waterfalls can be visited from the Argentine (2 days) and the Brazilian side (1 day). More on this on the detail pages.
Another highlight of the region is the Itaipu Dam, a joint project between Brazil and Paraguay. It is the largest energy producer on earth. Only the 3 Gorges Dam in China has the potential to generate even more energy, but has not reached it yet. The visit is possible from Brazil or Paraguay. More on this on the detail page.
Getting to Brazil is very easy by bus. Paraguay can be reached either by bus, which runs through Brazil beforehand, or your go from the little harbour in Puerto Iguazu by ferry, which crosses the river.
Each country has a landmark stone where Argentina, Paraguay and Brazil face each other. The landmarks in Argentina and Paraguay can be visited free of charge. There is a regular light and water show on the Argentine side in the evening. On the Parguayian is only the stone and nothing else. Brazil knows how to make money out of everything and even has an entry fee if you want to go to the landmark.
I would recommend 4 full days in Puerto Iguazu
Daytrip to the Iguazu Falls in Brazil
Daytrip to the landmark and Itaipu dam in Paraguay