For folk horror plus the outdoors in one night, an outdoor screening is hard to beat: a projector, a white sheet, blankets, and a fire pit if that's allowed. The Wicker Man (1973) and The Witch both suit a cold wind at your back.
Other ideas in the same spirit: a lantern walk through woods, a harvest fair or corn maze after dark, or a bonfire with roast chestnuts and mulled cider. If you're anywhere near one, a Samhain fire festival is about as folk horror as real life gets (Edinburgh's Samhuinn on 31 Oct is the best known).