Patterned Landscapes as an Art Sub Lesson
Here's a simple lesson that was done in middle school, but could so easily be done in elementary art too.
Mrs. Hare at Becker Middle School gave her students handouts with sample patterns. They filled their drawing paper with lines for foreground, middle ground, and background. Then they filled in the areas with pattern.
The supplies used were simple: paper and Sharpie.
The students had previously studied landscape, so they were acquainted with the basic principles of drawing one.
As a sub lesson, you could provide the teacher with the preprinted pattern sheets; a sheet showing the steps to draw the picture; and a finished sample drawing.
If the students are relatively independent, they could follow the instructions on their own. Most any sub could also demonstrate how to draw the lines across the paper and label them as foreground, middle ground, and background. Then, the sub could show the finished sample and get the students started.