This is the kind of writing where you just write for the act of it. In the context of note taking, fleeting notes is a type of this writing. When you write to engage, to get your mind to participate to the words on the paper or screen that are in front of screen on your very eyes. For this said purpose, doodling, sketching coud be seen as the same activity.

The more physical the activity is, the better. Pen and paper would be exellent. I myself like to draw the words on screen with a tablet, for it is immediately available and the physical engagement feel relatively on par.

Starting to write these kind of one-off notes should feel frictionless and tools to write should be readily available. And they are not for permanent storage. You should regularly process them, extract the todo items or filing them into some sort of an archive system, then throw them out once done.