I've been seeing a lot of Zentangles on the web lately. I've even seen them transformed into art lessons on Artsonia and I think this could turn into a really fun free art activity for the kids.
The definition from zentangle.com is as follows:
Zentangle is an easy to learn method of creating beautiful images from repetitive patterns. It is a fascinating new art form that is fun and relaxing. It increases focus and creativity. Zentangle provides artistic satisfaction and an increased sense of personal well being. Zentangle is enjoyed by a wide range of skills and ages and is used in many fields of interest.
Another site, TanglePatterns.com, has great directions and easy to follow directions to get started.
Here are a couple examples from Life Imitates Doodles