This assignment has worked very well for me for middle school and high school. It serves as differentiation, and also a time filler. I usually give students an 18x24 piece of either heavy paper, or tagboard, depending on storage options.
This gives students an opportunity to not only draw what they are truly interested in, but feel like they are building skill at their own pace and level -- while it also gives you as the teacher an opportunity to get to know students on a deeper level through their art, and give them guidance on drawing technique in the areas they need it most. On the back are many idea starters for students who have "artist's block."
This is an ongoing project and is meant to be either semester long or year long. Teaches students to not only slow down and fill the entire paper but also how to work at something and help balance their desire for instant gratification thanks to our tech heavy culture.
On a day when you are too suck to write real sub plans, knowing the students have this ongoing assignment to work on while you are out is a beautiful thing. And to them, it's exciting because they get a bonus day to work on their "NED'S."
A must have!!