I live about an hour and a half away from Sioux Falls and most of my extended family lives there, so I visit the falls quite often. It is always very beautiful. Recently the city has added nice paths so it is stroller and wheelchair accessible, and is much more easily navigable. Although it was much more fun when you could climb across the river on rocks, explore the queen bee mill, and hike along the banks, it is much more safe and family friendly than before! (When my dad was younger he used to swim there and slide down the rocks like a water slide--pretty unsafe as the water is swift! You are not allowed to get in the water anymore.)
I love walking across the bridge and watching the falls move while getting the misty breeze. There is a nice observatory tower, and the city always has it looking nice (including Christmas lights in winter, a light show on the water, etc). Once I believe they had a Shakespeare play inside then mill which would have been a really neat setting. If you are in Sioux Falls, be sure to make a stop at their namesake falls! It is a beautiful place to take senior pictures, engagement pictures, prom pictures, etc.