Güira Oga is a wildlife rescue centre on the road between Puerto Iguazú and the falls, caring for native animals that have been injured, orphaned, or confiscated from illegal trade. It is a quieter counterpoint to the force of Iguazú Falls, with the focus on conservation rather than spectacle.
Visits are usually guided, which helps because the animals and their backstories matter more than simply ticking off species. Go with realistic expectations: this is a rescue and rehabilitation centre, not a polished zoo, and that makes it more interesting.