The reason most travellers pass through Ciudad del Este is to visit Iguazu Falls, a short bus or taxi ride across the Friendship Bridge into Brazil (the Brazilian side offers the panoramic views; the Argentine side, reached in about an hour via a second border crossing, puts you right on top of the falls). Most visitors stay on the Brazilian side in Foz do Iguaçu and make both sides a two-day trip.
The second essential day trip is Itaipú Dam, 15 km north of the city, with free tours and a visitor centre. Combine it with the Itaipú ecological refuge and its small zoo of animals rescued when the dam's reservoir flooded the forest.