Dark and Stormy Night:
Wear a black sweat suit, black face paint, black spray painted hair, piece of cloth with gray cotton stuck on it around your shoulders. Use cardboard cut into lightning shape covered in tin foil attached, and a water gun, for the rain.
Thanks to Jenny for this very unique idea!
For a Starry Night:
Wear a navy blue sweat suit, paint face blue and add stars.
Thanks to Maggie for this additional idea!