I used a discarded blank flipbook from work as a miniature sketchbook. We give out free mini flipbooks in our animation exhibit - every once in a while kids will start a flipbook and abandon it with only a few scribbles. Sometimes I rescue them and use them as miniature sketchbooks I did that (Here) for my mini-bird-book.
Yesterday while cleaning my desk I came across an old one - take a look
It's real small, maybe 3 inches? I only filled these 4 pages, I should try to finish it - it would be a fun challenge. Maybe carry it around in my back pocket?
Here are some bonus rando-sketches I unearthed in the junk pile as well.