Onondaga Cave State Park is about 2 hours SW of St Louis. There is a campground and a Caverns you can tour, which is nice on hot days. The park is located on the Meramec River, so there is also swimming, fishing and canoeing, and hiking along the bluffs. The campground has basic electric/water campsites. They also have family campsites, which are a little larger. There is a playground and a small store.