I’m glad you mentioned Wall Drug, as I was ashamed to admit stopping there (TWICE! on the way out, AND on the way back)[:I]
My kids thought it was the greatest place on the planet!
I forgot to mention the Little Bighorn Battlefield NP. Like the Sunken Lane at Antietam and Devil’s Den at Gettysburg, it’s one of those places where bad things happened and the hair on the back of your neck stands up.