Here's a blind contour drawing i did in a cab recently. It's a good way to break out of your usual logical way of drawing. If my science is right, it shld strengthen the visual "muscles" in the right (visual/creative) side of the brain. That's because it forces the brain to only use the visual muscles and you totally ignore the logical side of the brain. Since drawing things accurately is closely linked to visual observation, it's a very good way to improve your drawing skills. I recommend drawing in unbroken continuous lines when doing this. It also improves you confidence in drawing.