There is a bus station in Puerto Iguazu and buses leave every hour or so for the Argentinian side of the falls and were fairly cheap. You have to pay an entrance fee for the park and can get optional boat tours of the river below the falls and above them. I did both and it was worth it. The Argentinian side has walkways and bridges that take you right out over the falls and over the Devil's Throat which is the enormous 3-sided waterfall. All in all I think what you can do and see close up on the Argentinian side is better than the Brazilian side but the view of the entire falls is better from the Brazilian side.
Some further points & pitfalls:
- If you use a US passport to enter Brazil (even if only for a few hours) you will need to pay a visa fee which I believe is USD $135. I think Canadian or Australian passports pay too. European Union and most other countries do not pay a fee. So if you have a choice (dual citizenship) on which passport to use you can save money. The same is true for Argentina. FYI I think the visa is good for multiple trips over at least 5 years.
- The boat trip under the falls gets very wet - it is like being in a strong warm shower. So wear a swimsuit or shorts, have a rain coat and if you bring a camera have a ziplock bag to keep it in.
- The walkway over the falls gets wet too at the Devil's Throat - like a rain storm - so you will need rain gear there too
- The river has a lot of whitewater so if you the kind of person who gets seasick or is afraid of powerful waves in a small boat think twice before doing the boat trip. Also hang on to your camera!
- The water level can vary dramatically depending on the time of year - during wet season the falls are much larger - in fact some years there are floods and the walkways are closed. I believe wet season is approx November - March but you might want to check.
- There are lots of bugs here so bring strong bug repellant and reapply after you get soaked.
- There is so much to see you could easily spend 2 or 3 days at the Falls and in Puerto Iguazu. There is a museum at the Falls and lots of wildlife. So having spent all the money getting there don't short change yourself by trying to fit it all into one day. In addition you can not guarantee sunny weather on any particular day, so again from this point of view it helps if you are in town for several days. It is much more fun seeing the Falls when it is sunny.
- It is easy to get both pesos and reals at ATMs in respectively Argentina and Brazil.
- However it is not so easy to change peso back outside of Argentina so don't get much more than you think you will need. (I had to go to three different Cambios at the Sao Paulo airport before I found one that would take Pesos and even then they didn't have a good rate). I recall that you need a government form in Argentina to convert Pesos back to dollars so keep your Cambio and ATM receipts if you plan to convert money back on leaving.
- I found snack shops but no major restaurants at the Falls so if you want more than moderately expensive sandwiches and pizza to eat bring your own food.
- I visited Iguazu Falls last year starting from Este in Paraguay. I took a taxi to Puerto Iguazu (there was a bus but after waiting 2 hours at the bus station and asking around and not finding it I got a taxi for about 200,000 guinis (or USD 50)). On the way back it was easy to find a cheap local bus to Este. But be warned it was very crowded and if you didn't tell the bus driver specially he did not stop at passport control (locals don't have to use a passport to move around the 3 countries). This can cause problems when you exit or reenter the country later without the correct entry and exit stamps.
Enjoy the Falls, they really are one of the wonders of the world!