A notebook that lays flat
I'm a fan of computers. Taking notes on them brings a lot of benefits: easy backups, syncing across devices, search, and invisible revisions. But for thinking through problems and tracking my daily thoughts, I've recently moved away from using note taking apps11 Like iA Writer or Obsidian..
Writing by hand helps me organize my ideas and avoid the tendency to get bogged down in revisions. Handwritten notes in meetings aren't susceptible to a computer's distractions. Using a pen, they're also immutable and capture how your thoughts evolve over time.22 To bring that to my computer notes, a script commits my notes to version control every 5 minutes. But it's not the same and way less legible. It's also fun for me to work on my handwriting and flip through pages of a well-used notebook. That's probably the biggest reason I've switched: I just like them more.

I've gone through a lot of notebooks that don't work well for me:
Hardbound notebooks (e.g. Moleskine and Leuchtturm) are fine for the first dozen pages but soon stop laying flat and require "breaking" the binding to work the book in.
Spiral-bound notebooks (e.g. Marumans Mnemosyne) lay flat but their wires get bent and caught up in a crowded bag. Because there's play in the holes for the wires, pages on the inside can get bent on the edges when the cover is offset.
Disc notebook systems (e.g. Circa) have all the drawbacks of spiral bound except they're very durable. I used these in college and generally liked them because they were cheap to refill and I needed a full letter size page for taking notes with diagrams.
Notebooks with no spine (e.g. Field Notes) don't lay flat and can't fit much paper in them, but they're very portable.
However, I finally found a style of notebooks I'm happy with. I bought a JetPens Kanso Noto at the end of last year and started using it as a journal. This is a lay-flat notebook with a mostly exposed spine: instead of a continuous cover, there's only some tape to hold the two cardstock covers together.

A5 notebooks are the best size for me. There's enough room on the page for readable paragraphs at my typical handwriting size. It's also portable enough to throw in a small bag or front pocket of a backpack.
I use a LAMY Safari fountain pen with a fine nib and De Atramentis Document ink in black. The Kanso's Tomoe River paper is pretty thin but handles it well. There's a little ghosting on the other side of pages, but it doesn't hurt legibility. Another color can be used for emphasis and makes it easy to spot edits. The cyan color of this ink bled through badly, which was a shame, but black and red are fine.

It's not as durable as a hardbound notebook, so I eventually wrapped it in a BKxAP Canvas Cover33 I guess this would address my issues with spiral-bound notebooks, too.. The canvas is soft enough to still let the notebook lay flat. It helps keep the sides of the notebook a little more protected since its own cover is exactly the same size as the pages. With thick seams pushing underneath the thin pages, I had to add a clear writing board to make it easier to write on the first and last pages of the notebook.

When I fill this up44 After 2 months I've used about a third of it., I'm excited to try two other options, in this order:
Pith Supply Yuzu55 Unfortunately this isn't readily available in the US.: Steven Schultz on YouTube claims this is the perfect lay-flat notebook and the colors look really nice.
Midori MD Notebook: It has these little perforated tabs in the corners to mark your location in the book without a fabric bookmark.