Along with Buenos Aires and Patagonia's Moreno Glacier, Iguazú Falls is one of Argentina's top three attractions, and one of the places people most ask me about. To be sure, it's a spectacular sight, far wider and higher than Niagara, with volumes more water, but I've grown to dislike its Disneyfied commodification--the approach to the falls feels like "Iguazú Theme Park." Away from the falls, in the rest of the surrounding national park, there's plenty of wild subtropical forest.
Still, anyone seeking a really unique experience, without the crowds, should travel to the downstream Iberá marshes of nearby Corrientes province. That's where you'll easily see fauna like the yacaré negro (black caiman) up close and personal.
Even if you don't seek them out, maybe they'll come to you. Today's Clarín carries an account of a five-foot caiman that parked itself on a family's doorstep near the port of Reconquista, across the Río Paraná in Santa Fe province. Responding to the attempted home invasion, local coast guard officials cautiously detained the reptilian perpetrator, then deported it to its natural habitat.