I recommend Graffiti. Its the definitely the best form of publicly improvised art. Even if you just do it in a sketchbook out in public it will increase you willpower for producing art straight out of your mind with somebody looking over your shoulder . It works effectivley because these laymen are baffled and dumbfounded by it, and they can't really say anything when they watch you do it because it is so unpredictable from what your doing with it, to where you're going with it and how you're coming up with it. They just stare in awe wishing they had the talent.
Strange coincidence that every man whose skull has been opened had a brain.