For portable note taking and thought recording which does not depend on modern technology, one ancient device which is still being produced and used by millions of people all over the world for scribbling stuff down, is the old-fashioned wood covered carbon pencil. Besides it being inexpensive, it is dependable (it never dries up or leaks in pockets or containers). It writes in any position using the technique of abrasive transfer. You can erase unintended mistakes and then write the intended thing down in the same place. you can make words, draw figures, create new characters and symbols which no keyboard can easily and quickly imitate. You can change size of characters easily and innovate writing styles as you go. You can stuff the written document in a pocket, staple it to a wall, fold it up and stick it in some container, you can read the document, even if it happens to have been soaked with water or a solvent. 

There are other advantages, but for versatility, universal availability, and extended usefulness, you can not easily secure a more used and useful writing instrument. Even though a wooden pencil must be sharpened after it has been used a while, devices for sharpening them can fit easily in your pocket or other carrying device. When not available, simply use a second pencil when you wear the first down. While other handwriting instruments can do some of these things, they can not do all of them. You can find a lot more on note taking at:  IsAnythingBetterThanPaper