The hike to Harney Peak from the Sylvan Lake trailhead  (trail #9) is one of the best in Custer State Park . Moderate in difficulty, it is a six-mile round-trip hike that provides spectacular views of both the plains and the hills. The peak elevation is 7,242 feet, the highest point in South Dakota and, for that matter, the highest point between the Rocky Mountains and the Eastern Seaboard. Expect to take 4–5 hours to hike the trail round trip.
To find the trailhead from the entrance gate, drive past the park store located at Sylvan Lake off of Highway 87 and take your first left. (This would be a right onto the road if you are traveling Needles Highway  and are entering the lake area from the south.) There is a parking lot at the end of this short road. Park there and cross the footbridge to the swimming area. To your right is a trail information board and the start of the trail.
Trail #9 is marked by blue diamonds. The trail is well traveled and at the start is wide enough for two to hike side by side. The trail was closed late in 2009 for a short time to allow removal of a large number of trees with pine beetle infestation. The tree removal is noticeable but doesn’t affect the overall scenic impact of the hike.
The trail begins in Custer State Park  but crosses into the Black Elk Wilderness Area, where no clearing will have occurred.
The elevation climb is constant but not steep in the earlier sections of the trail, and small forest meadows and marshes glitter in the sunlight that filters through the pines and spruce trees in the area. Small streams of water cross the path and as the trail progresses large rock formations loom to the left and right.
As the elevation increases along the way, more and more granite outcroppings become evident and the sky opens up. By the time you reach the Black Elk Wilderness area, the trees are thinner and the views encompass great distances.
When you reach the peak, what you see is a 360 degree panorama. The views extend past the Black Hills  into Wyoming to the west, and as far as the eye can see over the plains to the east. Harney Peak is crowned with a fire tower that was built entirely of stone by the Civilian Conservation Corps in the 1930s. The tower is open for exploration though it is no longer manned.