Matt Mets used paper cutouts on both the inside and outside of a lampshade to create a cute “monster after dark” effect.

This kind of fun with silhouettes is ripe for all kinds of customization: A crescent moon with bats that appear at night, a castle haunted by a nocturnal ghost, and lots of other now-you-see-them-now-you-don’t options. A reverse effect might also be possible, where an innocuous-looking doorway could have spooky eyes glowing from within. (The only caveat might be that the cutouts for the eyes may still be visible when the light is off.)

You could do something a little more permanent using stickers or even paint, but I kind of like the idea of temporary paper cutouts that can be changed as the whim suits you.