As most of the falls are in Argentina, you get to see then up close, so that you are often showered with spray. There are several walking tracks, and a free train service that links the trailheads to the park entrance.
From the Cataras station there are the Upper and Lower trails that lead you along the top of the falls, and around the base to reach the (free) boat service to San Martin Island.
From the Devil's Throat station you walk out along a causeway to a big horsehoe shaped section of the falls, where the noise of the water is deafening, and clouds of spray come drifting upwards to soak you.
This has to be an experience not to be missed. But to complete it you need another half day to cross over to the Brazilian side and see the full panorama from over there.