Here’s a simple one to lock away in your brain for those “fill-in” times, or for the substitute. In fairness, I saw a version of this painting on Pinterest, and knew it would be a great school activity. My seventh graders loved it.
Ink pen/permanent marker
Begin by sending washes of color dripping and rolling down the page. Have fun! Blow the drips into splits and twists! Wait for one color to dry (or not) and send another down, creating layers of varied colors.
Mixing up a good dark color, paint in the idea of a forest floor on the side the drips began. The drips now look like spikey, wild trees in a fantasy forest.
When it is fully dried, take a pen and sketch in a few forest monsters, and maybe a couple of squiggles to mark the floor. Keep it simple.