As a resident of Spearfish, this is my go-to for summer weekends. I pack up my lounge chair, a good book, fishing poles, and head out to the beach. Or if you're lucky, find a friend with a boat and troll for walleye. One side of the Reservoir is a developed day/camping area, with paved roads and individual camping spots. The other side, we locals call it the "free side", has a fabulous sandy beach (named Tequila Beach) and a lot more shade. However, the free side is only accessible via a washboard-gravel road, and there is no guarantee you won't be sharing your space with a few other families.
Photo courtesy Belle Fourche Reservoir