Visit both sides and see the differences
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You have to see the waterfalls of Iguazu Falls from both the Brazil and Argentina side. That can easily be done in one day, so that you can visit these impressive waterfalls at their best. The day will probably be one of the highlights of your trip.
You can combine the thrills in Argentina in one day and take special overview photos in Brazil. I started in Argentina, mainly to be ahead of the many other tourists at the various viewpoints.
Small islands can be seen between the many waterfalls. Also pay attention to the old bridge that is still in the water. They are the remains of a great flood. The walk on the water on the Argentinian side with the edge of the waterfall for dessert is unique and also confronting. It's no place for fear.
In the afternoon you can visit the Brazilian side at your leisure, and take a good look at the waterfalls from a distance. It gives you a nice overview of all the water. Coatis are regularly spotted, so take a good look around you.
The border formalities in both countries are limited, you will receive a visa for a day (for a small amount). Logically you have to go back on the same day. But that is not a problem. Not even when it is very busy. Although it is smart to avoid weekends and school holidays.