When I'm out having a cup of coffee I usually grab four or five extra paper napkins to make use of while I'm sipping. Sometimes I'm making lists of potential blog topics or, if I already have a subject in mind, I might outline or list items I could include.
Other times, I might sketch car designs or poses nearby people assume. And I've found that fine-point ball point pens work just fine on napkins provided there is more than one napkin layer (some cushioning helps prevent the pen from gouging through the paper).
These drawing are small, seldom exceeding two inches (5 cm) in the longest direction. And because they're small, I can't get hung up with details -- a good practice that counteracts tendencies to make images more "complete" than they should be.
Harry Beckhoff (1901-1979) was an illustrator who worked in thumbnail sketch mode. He didn't make large, sweeping-gesture sketches and then boil them down to production size. Instead, he had his thumbnails enlarged and then traced them as the basis for the final job.
Leif Peng mentions this unconventional practice in this post about Beckhoff. Another take on him is here. Otherwise, there seems to little information about him on the Internet.
I find Beckhoff's work charming, and hope you too will like the following examples.