The Badlands are beautiful, vast and forboding. On one visit we took advantage of the helicopter tour over the area. The view was breath-taking. It's been a while and I'm not so sure of my memory but the cost was somewhere between $10 and $15 per person for the helicopter and the ride lasted about 5 minutes, so take this into consideration if you choose this activity.
There is a free walking tour of an area chuck full of fossils. This was pretty interesting as there were glass display cases with explanations of what we were seeing.
The coloration of the earth strata was gorgeous, especially as the evening sun cast its rays along the hills. No matter the beauty, this is a place where you don't want to wander off the beaten track.