Great State Park in the St. Croix Valley
Lots to do here. Hiking, Mountain Biking, bird watching and Campin! Three great campgrounds with the 300 loop being the oldest. However, it is the only one with lake sites and has a night playground in the center of it for kids. Great trail from the campground to the falls as well with very little elevation gain. 200 and 100 loops are newer and the 100 loop has the best shower facility as well as a small playground. The draw back for 200 and 100 is more road noise (Cty A) and less tree cover. Cell service is solid 5G in the 200 and 100 campgrounds. 300 isn't quite as good as you are in the valley more and have more trees.
We've camped here in every season by winter and at multiple sites.