When creating a sub plan that needs only simple instructions and materials, Klee's painting, "Cat and Bird," might be just the foundation upon which to build.
Paul Klee Idea Emphasizing Shape, Pattern, and Warm and Cool Colors
The teachers at Jamestown Elementary, Ms. Worden and Ms. Teitsma have written a blog post that suggests rich resources from which to teach this lesson.
Since my older students would often ask if they could do a lesson that the little kids did, I know that sometimes they like the simple lessons too. The perfect time to let them do one of those simple lessons is when they have a substitute teacher in art.
Check out this lesson at Jamestown Elementary Art Blog.