At 7242 ft, Harney Peak is the highest point in the U.S. east of the Rockies. You must bag this peak!
The historical fire lookout atop the peak is a must see. 360-degree views await. Use the old iron "windows" to frame your photos of the beautiful Black Hills.
Consider taking a long way to the top. I'd recommend hiking through the Black Elk Wilderness, perhaps starting at the Willow Creek Horse Camp. There are numerous interesting rock formations, lots of colorful wildflowers and lichen, and sweeping views of the peak and its environs along the way. Make a day of it!