My 21 mo son loves to draw on his blank spiral bound sketch pad with his washable markers - many times a day. At first, I regulated the markers by letting him choose one and then replacing the cap when he finished with it before choosing another color. He started replacing the cap and now manages all 11 markers by himself.
I also let him play with finger paints sometimes. I tape down a big piece of freezer paper and squirt some blobs for him. He's not a big fan of dirty hands, so this does not last too long.
I think its important to let him do his art freely. I do not interfere or instruct him (except to tell him that markers must be used on the paper). Just observe, inquire, and praise. Its awesome to watch his drawings progress from single color scribbles to multi-colored shapes and various lines.
We also play with play-doh together. My son is gluten intolerant, so I found a recipe for gluten-free play-doh online. He likes to lick it.
Have fun!