I bought a Fisher Backpacker Space Pen to use on short hikes.
If you combine it with a Rite-in-the-Rain notebook you can jot down your flashes of brilliance while doing pretty much any outdoor activity, with the possible exception of scuba diving. The pen will work under water, but I’m not so sure about the notebook.
They certainly didn’t lie about it writing under some arduous conditions, but the little blighter feels too small in my hand.
It’s kind of like using a pencil and wondering how many more times you can sharpen it.
Beyond that, I highly recommend buying one if you’re headed outside. They fit in your shirt pocket, and they write under water, be it the ocean, a spilled canteen, or that sweat dripping off your nose when it’s 110° in the shade.