Peak Season: Summer (but it's rarely busy)
Off-Season: Fall - Spring
Cottonwood Campground (South Unit), Juniper Campground (North Unit)
I liked the South and North Unit quite a bit. The Southern Unit is larger and boasts several trails ranging from hikes along dried up creeks (Paddock Creek and Jones Creek Trail) to uphill climbs that give way to beautiful vistas of the surrounding badlands. The Northern Unit follows along the Little Missouri River just like the Southern Unit but it is smaller but has more geographic diversity. Rugged mountaintops turn in to rolling grasslands and the park road ends at a view of the Little Missouri River at Oxbow Point.
The campsites are quaint and scenic. Both units have an more than enough sites to handle the relatively paultry number of visitors that stay at these parks. If you're lucky you'll get one of the sites nestled up against the Little Missouri River.
The prairie dogs are hilarious.