+1 to sketchbooks. I started using them in ~2014 and have never stopped. The tricks for me were to fold each page in half to get more columns and restrict my wandering handwriting/doodles, to find cheap books (which still have hard covers) at a local art store that serves students, and to make sure I always write the date/time down when I start a new train of thought, with a heading.

Every few years I go through the last 12-20 notebooks and take pictures, then get rid of them. I don't go back through them that much, really, beyond things more than ~6 months old, but when I do I feel like it's a goldmine.

Digital notes are much more useful for getting long term value out of them, I found. Paper notes are fun, and it's much easier to sketch things freely and just doodle on them.

Paper sketchbooks are also really useful as temporal artifacts. Looking back at them, I know where I was, I discover things I have forgotten, they have different formats, they mix life with work with hobby. I don't think the digital notes will have the same nostalgia factor.