"I'm All Done" projects?

[Drew Pitchers]

Get your hands on some art term word searches. Then require your students to highlight the found words by shading value scales over them in colored pencil. There is even a "bonus" color mixing aspect when this is done correctly.
Word searches are usually worthless, but practicing shading while becoming familiar with vocabulary for your class is win-win no matter how you look at it.
Plus, you can change the rules to have the crosshatch the words and other techniques depending on what you are covering in class.
I've been doing it for ten years. It keeps them busy, but they don't love it so much that they rush their actual work.
I sometimes copy a puzzle or maze to the back as well...but I don't require or award extra credit for it.
If you don't want to find your own, search for mine by typing "art word search" on Teacherspayteachers. It helps if the word searches are oversized to allow more shading.

[AMC27]

I agree with those saying sketchbooks to draw in or art books they can look at. The sketchbooks could be a place where they can draw anything they want without the guidelines of a formal project and art books will keep them focused on art making and introduce them to artists they might not have ever heard of.