The park is huge with three different hikes that allow various views of the many falls: the superior circuit, the inferior circuit, and a short train ride to the devil’s throat, which I recommend doing in that order. Words cannot describe how beautiful these falls are; there are rainbows all over the park, a number of waterfalls, lush green forest, a variety of animals wandering about, and views that will take your breath away. Entry to the park is 500 pesos, which is about 30 US dollars, and while it seems like a lot, it really is completely worth it. You will never want to leave.
I stayed in a hostel in Puerto Iguazú, which is about a 30 minute bus ride from the park and filled with a number of great restaurants. It is also a 30 minute walk to Tres Fronteras, the meeting point of Argentina, Paraguay, and Brazil. There are two rivers separating the three countries, but the view is beautiful and it is a great place to pass time with friends and, if time permits, watch the sunset.
If you are in Argentina and you have the time, you really must visit Iguazu. It is without a doubt, one of the most beautiful and breathtaking sights you will ever see.