I have a little notebook which I keep some of my ideas in, so I can go back to them when I feel bored.

Obsidian

Same here. I really like the Excalidraw plugin for drawing diagrams when the occasion calls for more visualization than the pure markdown allows.

I have an org-roam knowledgebase, and I keep a paper notebook when I'm away from my computer which gets input into the computer as soon as I'm back.

Google Keep at the moment.

I've tried Obsidian+SyncThing, custom DIY apps, turtl, simplenote, Zim, probably others.

Nothing I tried was quite as frictionless on mobile as Keep, and the wear OS tiles are great.

A nice notebook A pen

I honestly tried most of the note taking apps, none really worked out for me, I always end up using my memory directly, with a mix of few large txt files synced in my drive.

One that I wrote for myself...

https://github.com/cu/silicon

After extensive search and evaluation for a decent replacement of Ecco Pro I finally wrote a tool myself many years ago which helped me to keep track of all my projects since then: https://github.com/rochus-keller/crossline/

Contextualise, a tool I personally developed: https://contextualise.dev/

Contextualise excels at organising information-heavy projects and activities consisting of unstructured and widely diverse data and information resources.

Contextualise is open-source: https://github.com/brettkromkamp/contextualise

I just use a plain git repository with markdown files, structured hierarchically. Total freedom of how to organize things, and I can work with the tools I like (git/terminal text editor).