Back in the days of film, I used to have lots of fun with double exposures. The principle is simple enough: take a picture, do not advance the film (usually requires you press down a button to disengage the sprockets while you arm the shutter), take a second picture on top of the first.
Even though the principle is simple, taking a good double exposure requires some creative thinking. Just exposing two bright pictures on the same frame would result in over exposure. So what you want to do is find some dark areas in your fist exposure to expose your second exposure in. If necessary, turn the camera sideways or even upside down for the second exposure. And, of course, you want the resulting double exposure to look good, mysterious but good.
Here are some examples and see if you can figure out they were done (either in camera, or in software): bitrebels.
