I carry at all times a tiny notebook (smaller than my hand) and a pen so small I can barely use it comfortably. That's low tech and not very efficient (because I'll need to type it up later), but very quick, easily survives the sometimes inhospitable pocket environment, doesn't need electricity and works for non-textual thoughts.
I do this too, and it holds my former wallet contents in a pocket, so not even an extra thing to track.
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