White Water Walk takes you by elevator down into the Niagara Gorge, then along a boardwalk beside the furious Class 6 rapids downstream from the falls. It is not as famous as the boat rides, but it gives a much clearer sense of the river's power after the water leaves the brink.
The walk is short and easy, with viewing platforms close enough to feel the spray and hear the roar. It is seasonal, so check Niagara Parks before planning around it, and combine it with the Whirlpool Aero Car or the gorge trail system if you want a fuller half-day.