In the creation of the Itaipú Dam, the dam company was obliged to set up a series of eight private reserves that now protect the last remnants of the Alto Paraná Atlantic Forest. Three of the most interesting are Refugio Tati Yupi, Reserva Itabó and Limoy. To visit, you'll need your own vehicle and prior permission (permits take a while to be approved). Basic accommodations are available free at some reserves, but you'll have to bring your own bedclothes and food.
Refugio Tati Yupi, 26km north of Ciudad del Este, is the easiest to visit. A popular weekend spot, it is best visited during the week on a day-trip from Ciudad del Este if you hope to see animals. Serious animal-watchers will prefer Reserva Itabó, 100km north of the city. The forest here is undisturbed and teeming with wildlife, including friendly armadillos and endangered Vinaceous Amazon parrots. There are good accommodations, too, though occasionally they're occupied by school groups. The largest reserve is Limoy, another 65km further north, but the facilities are basic and access is difficult without a 4WD.