Origami
During library time as a kid, I rushed straight to the 700s (arts) section every single time - origami books, how-to-draw books, puzzle and riddle books, Magic Eye books. Of all of it, origami is the one that stuck. I got genuinely good at it as a kid, and it's stayed a quiet, portable hobby ever since - the kind of thing you can pick back up any time with nothing but a square of paper.
It's carried through some meaningful moments too. I folded pieces for my wife while we were dating, and she still keeps them displayed at her desk. Now I've picked up a step-by-step book to pass the skill on to my son, so he can learn it the way I did.


