When I taught middle school art, one of the sketchbook assignments I had the kids complete was a page of doodling. They looked very much these. Bohemian Shadows blog gives a nice tutorial for this type of doodling. There are also books and workshops with the name "Zentangles" (not by Bohemian Shadows).. they coined the name which is basically just a glorified name for doodling. Anyways.... I think this year I will print out some examples for my elementary students for them to work on when they are finished with a main project. It would be cool for them to make these as an ongoing project.. maybe a book of sorts or a large fold-out artwork?
Bohemian Shadows: zendoodling