Are you a musician, or do you know any musicians?
My Moleskine music notebook is one of the two most valuable tools I have. I’ve got a bunch of different music notebooks, but this is the only one I carry around with me—the hard cover protects the interior, and the paper inside is nicely printed with light, thin ledger lines. Indispensable. The one above fits into a bag or backpack, there’s also a pocket size version that you can carry in a jacket, or cargo pants.
Of course, you need a writing instrument. I use and strongly recommend the Tornado Stealth mechanical pencil.
I’ve been using one for over three years, and I’ve used nothing but this in all that time. Very sturdy and well-made (it’s metal, not like so many plastic ones) with a nice thick, soft lead that does everything. I love this and have given it as a gift, and will keep doing so. If you don’t like the basic black, you can get various colors and designs, including one with Einstein’s formulas for getting to the equation that energy is equal to mass times the speed of light squared.