Ybycuí is 5,000 hectares of surviving Atlantic forest in the hills of Paraguarí, about 150 km south-east of Asunción, and it is the national park most people can actually get to. Created in 1973, it protects one of the last good fragments of the Bosque Atlántico del Alto Paraná, an ecoregion that has lost something like nine-tenths of its original cover in Paraguay.
The forest is dense and genuinely tall — cedar, palo rosa, guatambú — and the streams cut through it in a series of falls and pools. Salto Mina and Salto Guaraní are the two everybody swims in. There is a camping area, a short network of marked trails, and enough shade that this is one of the few places in lowland Paraguay pleasant in January.
The oddity is at the entrance. La Rosada was Paraguay's first ironworks, built in 1854, the plant that made the country briefly self-sufficient in iron and armed it for the war that destroyed it. Brazilian troops burned it in 1869. The furnace, the dam that drove the bellows and the workshop walls are still there, restored, with a museum in the old buildings. Industrial archaeology in a rainforest is an unusual combination.
Get there from Asunción by bus to Ybycuí town and then a local bus or taxi the remaining 30-odd km to the park gate, or drive. Weekends in summer are busy with Asunceños; midweek you may have the trails to yourself. Take repellent and expect to get wet.