Visiting the Iguaza Falls, both from the Brazil National Parks side and the Argentina National park side.
The majority of the falls are on the Brazilian side, but the Argentina side has the view of them, along with the boat tours up to the falls from the river.
The Brazilian side does offer a wonderful full moon tour during a 4 night stretch each month.

I stayed in Foz du Iguazu in Brazil vs Port Iguazu Argentina because I was already vacationing in Brazil on this trip. While these are small towns, there is still so much to do around them. Numerous other attractions including water parks (because getting waterfall wet is not enough?) a Hollywood car museum, Tango shows (Argentina side), South America's largest outlet mall just across the bridge into Paraguay, because this is the point where all three countries border one another.

I was there for only 2 days and 3 nights, and could've easily spent a full week exploring everything.

Music : "Once Again" from bensound.com
They have a lot of good music for all the feels.