I really enjoy my camp outs at Greenbush. Site 9 is on the purple trail which gives you access to the entire Kettle Moraine. I think the picnic area is also on the trail, or site 3 it's as well. I normally camp in September and October and take advantage of the Civil War reenactment and Halloween fun.
Park Rangers are excellent and not stuck up and pretentious like Devil's lake. The park is very large and navigation can be challenging. Don't be hesitant to chat up a ranger anywhere in this park. Professional and very knowledgeable.
This park has year round activity. Horses, snow mobiles, hiking, mountain biking, CC skiing and more. Greenbush puts you right in it all. I will also recommend the short drive to Elkhart Lake for the little bakery coffee shop downtown. Fun!!