I would say castle rock county park
Castle Rock County Park
There's castle Rock county park being the closet (nice but the sites are a bit more open) seconded by Castle Rock state park (much nicer). Also if you decide you need lodging (not camping) theres a couple places with cabins in the area.
Juneau county Park
Buckhorn State Park has great sites in the middle of the lake. Access is just north of Shipwreck. Ask around.
Thanks! Your answer is awaiting moderation.