When I was in school, our art teacher taught us a trick. You draw blobs on a page and then you draw vertical lines from the top to the bottom of the page. But you have to keep the lines parallel, going around the blobs. It takes a lot of concentration and it is amazingly calming. Your breathing slows, your heart rate drops and you stop worrying about stuff for a while. You become focused in the moment. It's very Zen.
And you don't have to spend money on a colouring book.