In my vacation sketchbook I used a compass to draw overlapping circles of various sized and inked them. Then I started filling them with a variety of doodles, working in pen.
I hid a few odd things in there: a few buttons, a balloon, a face.
When I was done I reinforced the outlines of the circles with a second line for more definition.
This was a lot of fun and an easy project to pick up and set aside to fit into a schedule of other activities.