Pencil. Usually a golf pencil or worn down short pencil about 3 inch long. I like a pencil because I learned form my dad that it has so many uses. It never runs dry, never leaks, what you've written won't run if it gets wet, can write on many surfaces, and can be scraped down for a dry graphite lube if needed. So light weight ands small it goes unnoticed.
Homemade pad to take notes on in my wallet. Fold up a piece of paper 3 or 4 times and put two staples in the top and trim with scissors to business card size and keep in wallet. This will give you a small pad with several pages. Use golf pencil to write notes.
Carl.
I always carry a 6B pencil stub, nice thick lead that won't break easy and leaves a dark line I can see without my glasses.
I use the reply envelopes from all the junk mail I get to write notes on. I carry a small diary too.
I also carry a small piece of chalk wrapped in masking tape to write on wood or walls if need be.
A piece of string (13') marked with a marker pen every inch for the first foot, then every foot after that, it's
also for tying stuff. I often carry the marker pen as well.
A peanut lighter, if I think I might need one, or a Zippo if I know I'll need one.
A magnifying glass on a cord around my neck as I hate carrying glasses.