While the Brazilian side of the falls can be explored in half a day, you could easily spend two or more days enjoying the Argentine portion of the park.
After settling into your hotel, grab a bus to the national park for your first exploration of the falls. If it's already hot, begin with the Garganta del Diablo or a boat trip under falls, for maximum spray. Relax afterward at Guira Oga, or take in the town's tiny Ecomuseum and Museo Municipal de Bellas Artes.
Begin the day with a quick trip to Brazil, where you can spend the cool of the morning photographing those panoramic vistas of the Devil's Throat and the Argentine falls. Unless you want to visit Itaipu Dam, or splash out on a helicopter ride, it's back to Argentina, for a half-price second day at the national park.
Consider booking a tour to Wanda Mines, with those remarkable amethyst geodes, and San Ignacio Mini, where you could spend the night before embarking on the next adventure.