I do some of my best work on napkins... Both building sketches and written ideas as well. Little square bar napkins are perfect... one for each of the major principle. This allows parallel trains of thought as I randomly think of items in stream-of-consciousness fashion on each of the subjects.
Here's another example. I was trying to work out some of the inner workings of the Original Green recently over lunch. I started by trying to get the three parts of the Original Green clear (workman, tool, and artifact.). The bottom of the top right napkin is a story explaining what the OG is. I've often used the term "wisdom," but realized that it's really shared meaning.
That began an entirely new train of thought, which went on the top left napkin. Wisdom vs. meaning. Expect me to tell some stories soon based on this idea. But that idea led back to a chapter I'm now writing in the Original Green that I was going to call "the Architects' Obstacles." as you can now see, it will be "the Architects' Achilles Heel" (or something similar.) in any case, it also deals with different aspects of shared meaning. And it sprouted another idea about the three characteristics that allow Living Traditions: they must spread widely, persist, and cause real change.
All four of these trains of thought can go on separate napkins, rather than being on any single "timeline" like what might occur in a Twitter stream, notes written in Word, marker notes on a whiteboard, or pretty much anything else that is sequential. Bottom line, if you want the ability to have randomly branching trains of thought, then the medium needs to exhibit the characteristics of napkin notes: branchable at any point, low threshold to branching (pick up another napkin from the stack; don't have to walk across the room,) allow back-notes on top of what is already written, cheap, and available on any Main Street (because you're not always in your office.) Personally, I can't think of anything that fits this bill except paper napkins. What else is there? There must be something else that does all this.