Doodling for adults is how I think of Zentangle®. I enjoy crafts of all sorts, and this is the sort of thing that I can do anytime, anywhere. All you need is a piece of paper and a pen. The idea is that you create abstract patterns of black ink on white paper, traditionally in a small square, 3.5 inches or 89mm in size.
Increasingly people are tangling in different shapes, and creating Zentangle Inspired Art (see pic below), building up a piece of art that often confuses the eye in its complexity.
http://tanglepatterns.com/tag/zentangle is a great resource on different patterns and how to draw them. Zentangle® is a registered trademark of Zentangle, Inc.
And maybe because I like Zentangle®, I like the following artist, who produces large, complex pen and ink pictures with patterns http://www.elihelman.com/