as an exercise, I do small paintings on scrap cardboard (and scrap metal) that I've gesso'd. My constraint is that I have to finish them in under an hour. It's basically a way to keep myself in the habit of painting regularly and not get too hung up on perfectionism.
So all of these are oil on cardboard. Usually somewhere in the range of 4x6 inches. I usually try to stick to two or three colors at the most so I don't spend all my time mixing paint.