Here is a free art sub PowerPoint lesson for you at my Teachers Pay Teachers shop: Art Sub Lessons. This lesson is one I use with kindergarten. We do lots of moving around making the shapes of lines as we look at the slides. TeachersPayTeachers requests that you make your free PowerPoints smaller than your "for pay" ones, but this is still full of good images and information for your students and subs.
Here are a couple of "easy sub lessons" from Dali's Moustache. One is a third grade lesson about Jackson Pollack's action paintings. The other is a fourth grade lesson called "crayon fireworks."
Another easy way to prepare for an art sub is to go to my shop, Art Sub Plans. I have created complete lessons in which all your sub has to do is read the slides and follow the directions. They are full of visuals and are created to be used with the most simple supplies. Pencils, paper and crayons are all your students will need to create imaginative works of art.
These are perfect for those emergency absences when you do not have the time to write detailed plans but want to leave quality plans for your subs.