The Upper and Lower circuits are the two main trail systems on the Argentine side of the falls, connected by elevated metal walkways that bring you face-to-face with individual cascades.
The Lower Circuit runs along the base of the falls and has the best up-close views — you are level with the water as it crashes down around you, and on certain sections you will get thoroughly wet without any additional help from the boat rides. The Upper Circuit follows higher ground and looks down onto the tops of the cascades, giving a different perspective on the scale of the falls.
Allow half a day for each circuit if you want to do them justice, or a full day if you intend to take your time. Both circuits start from the visitor centre area. The trails are well maintained and walkways are barrier-equipped, but wear shoes you don't mind getting wet on the Lower Circuit.