Zentangles have become quite the rage amongst my artist friends, so I thought I'd give it a try:
"Zentangle is an easy to learn method of creating beautiful images from repetitive patterns." They are traditionally done only in black and white, although many artists use them as a base for adding colour. You can learn more about it here. The repetitive patterns are called "tangles", and you can find lots of patterns here.
It is fun and relaxing, and I'll definitely try more. It feels like it might be a good airplane distraction, so I plan to do some on my upcoming trip to Spain. (I might be doing one while you are reading this.)