The Triple Frontier (Marco das Três Fronteiras in Portuguese, Hito Tres Fronteras in Spanish) is the point where Brazil, Argentina, and Paraguay meet at the confluence of the Iguaçu and Paraná rivers. Each country has built a small landmark on its own bank, and from the Brazilian Marco, you can see all three countries simultaneously — a mildly thrilling moment of geography. The Brazilian monument sits at the tip of a park south of Foz do Iguaçu proper, with a small plaza and vendors selling snacks and trinkets.
Sunset here is excellent: the two rivers catch the light differently and you get a sweeping view across to the Argentine and Paraguayan shores. Boat trips from Puerto Iguazú on the Argentine side will take you out onto the water between all three countries, which is a more dramatic experience than standing on any one bank. The Friendship Bridge, which carries road traffic between Brazil and Paraguay, is visible from the waterfront.