Badlands National Park in southwest South Dakota offers buttes, pinnacles, and spires in one of the largest prairies in the United States. It has the world's richest fossil beds from the Oligocene epoch and wildlife including bison, bighorn sheep, black-footed ferrets, and swift fox.
The Saddleback Trail is rated Strenuous. Wow. I made it all the way up to the pinnacle and back down again, with some difficulty. My wife turned around once it became nearly vertical. About 2/3 of visitors turn back halfway up this ridiculously steep and slippery trail. Do not attempt without good hiking shoes, water, and other proper hiking gear.
But if you reach the top, the view is wonderful.