Sylvan Lake is the prettiest easy stop in Custer State Park. Granite walls and boulders pinch the water into a small, photogenic basin, with a short loop trail that gives you the classic views without needing a long hike.
It is also a trailhead area for more ambitious walks into the surrounding rock country. In summer it can be busy, but it still earns the attention: the lake is one of the places where the Black Hills feel most like a mountain pocket rather than a forested plateau.