In class my students have been working on zen doodle.
I think this is a great way to let go and just play around with different line work and shapes and not caring what it "should look it". So pick up your sketchbook and start doodle today!
If you like it and want to explore more with zen doodling I have found two books that can be good tools for more ideas.
The Joy Of Zentangle
Zen Doodle Tons Of Tangles