I LOVE this lesson on Paul Klee from A Faithful Attempt! "Miss" has done an excellent job of laying out the background and process, so please visit her blog for the complete instructions. She has a number of additional excellent projects posted, as well.
Many of my 6th grade students said it was their favorite so far (we did it in early January), and the success rate was extremely high. This could easily be done with younger students, as well (3rd and up maybe?). It's an excellent introduction to Paul Klee, abstraction, and cubism.
I required my students to choose a color scheme for their composition: warm/cool, complementary, analogous, monocrhomatic, or triadic. A thin wash of tempera paint is applied over black colored pencil (for the lines) and construction paper crayons. The results are beautiful, and students got to experiment hands-on with the way color can change the mood of a composition. (Some layered a couple of tempera washes, going from blue to blue green or blue violet, etc., because their color combinations didn't turn out in reality the way students imagined they would. Great learning experience.)
I've done the project now with three of my four sixth grade classes. Here are a few finished pieces...