Here’s an oldie-but-goodie. I’ve been scribbling things using Mr. Doob’s procedural drawing tool Harmony for a few years now, and it has brought me hours of joy. Can’t get enough of it.
It’s quite simple; you have a drawing area & can set your background and brush colours. Select a brush, change its size, and enjoy yourself. The tool lends itself well to rapid sketching, and interesting shading experiments.
Here’s a tree drawn only using the ‘Ribbon’ brush:
Check out other projects by Mr. Doob while you’re at it. Incredibly productive individual.