This lesson idea is from "Ms. Brown's Art Class." I think it could be adapted for use by a sub.
Ms. Brown uses block tempera in her classroom. If I had a sub, I might use those same plans or either draw with Sharpie and then fill in with marker or crayon. She has nice instructions on her page. She also uses the book, "The Dot." (Thanks for all the lesson plans on your blog, Ms. Brown!)