In the extreme north-east of Argentina, squeezed in between Paraguay and Brazil, is the province of Misiones. The name is derived from a couple of Jesuit missions in this area. To these missions the Jesuites brought the Guaraní Indians to teach them the gospel. Compared to how the Indians fared elsewhere in Colonial America, this was a decent deal for the Guaraní, as they worked to serve themselves. In the 1760s the Jesuites were expulsed, and one of the reasons for that, was to get access to the labour craft. Today, only ruins remains of the missions. Some of these have been labeled world heritages by UNESCO, and the biggest ruins are those in San Igancio de Miní. They are unusual to be in this part of the world, but as a European I'm not sure that I found the ruins that special.
Today, the Guaraní is a small and not very significant group in Argentina, but they are still numerous in Paraguay, and Guaraní is an official language in Paraguay alongside Spanish.