high Andes. With a full week, I would hike the Inca Trail as part of this, and you might even be able to squeeze in a visit to Huaraz (a massive glacier-covered mountain). That would be my #1 choice in South America, easily.

Other great alternatives, though lesser options IMO, include the Galapagos Islands in Ecuador, Iguazu Falls on the border between Brazil & Argentina, Torres del Paine national park in southern Chile, Cartagena in Colombia, Easter Island (part of Chile, although a long way out in the South Pacific), an Amazon cruise in Peru or Brazil, and the Angel Falls area of Venezuela (haven't been there yet, but I understand it's amazing).