There are three major tracks to enjoy in this terrific park. The lower circuit (views from the bottom of the falls, 1.4 km long), upper circuit (top of the falls, 1.8 km), and Garganta del Diablo (train ride and 1.1 km walk). I recommend starting from the lower circuit so you get to admire the full length of waterfalls and get a little wet.
Then the upper circuit will amaze you as you will see how relatively calm river stream can produce such huge amounts of falling water.
If you get lucky, you will meet some capuchin monkeys around the trails.
Garganta do Diablo is the absolute highlight of the falls experience. A big hole where 3 different water flows merge into one abyss. The amount of water is so much that it creates a mist around the hole with birds flying in and out of it. And you are totally surrounded by waterfalls. What adds to the supernatural feeling of the place is that you can see the top of the rivers flowing above the hole. Magical! We felt a little like Alice in Wonderland where we stepped into a totally different reality.
Everything is more expensive in Argentine side of the border from bus tickets to the national park to the entry ticket itself. Bus from the border to waterfalls (cataratas) is ARS130 return and entrance to the park is ARS330. Remember to validate your ticket at the end of the first day – the second consecutive day entry to the park is half price.
Oh, and pretty much the only food available inside the park is overpriced subway. A foot long subway is ARS160. So be smarter than us and don’t feed the monopoly by being disorganised and ending up buying those on both days. On the plus side the water from the taps is drinkable so no need to buy expensive water bottles.
Should you have extra time on the second day – there is the Macuco trail. 7 km return walk through rainforest. Nothing special but nice to do if you have time and energy.