First, most campsites in Oregon are done via web based reservations, Oregon State Parks and Recreation
Second, you may want to check out Brown's camp. It is in the coastal mountain range between Portland and the Coast (on the road to Tillamook, Oregon). This is a state run off road area, and has everything from smooth dirt roads to a true bouldering trail. You can also drive on the beach in areas--Fort Stevens State Park, near Astoria, allows this.