I’ve always liked the effect you get when two lights are flashing at not quite the same frequency. The pair of lights goes through intervals where they both flash on and off together, and intervals where one light is on when the other is off.
Once, I was behind several cyclists, and all their back lights were flashing, at slightly different frequencies. I wondered whether you could arrange a set of lights in a grid, each light flashing at a fixed frequency, such that every now and then they came into phase and made a picture which flashed, alternating with its “negative”.
I played with this for a while, realised you can make two pictures, and used the idea to make a real grid of flashing lights in a wooden frame.