When you don't need to share information, there's nothing better than a notebook to see all of your ideas spread out and iterated. No need to log into anything. Take it with you anywhere -- sit on a bench and ideate. Go to the gym and jot down ideas.
I can keep my daily goals in a checklist in there and just check them off 1 by 1. No need for GitHub projects or anything like that because there's no need to communicate status with anyone.
Keeping track of the decisions you make, both to do things and to NOT DO THINGS (and why) is almost a superpower. You can save so much time when you come up with a 'new but vaguely familiar' idea by being able to check back through your notes and see what you came up with last time it occurred to you. Maybe circumstances changed, maybe they haven't, but you can bring yourself back up to speed on what you've done previously with little effort.
I tried to differentiate mine a lot, but I often found I had the "wrong" notebook at the "wrong" times. Now I have have a general notebook, and other notebooks that might live in 1 place. E.g. all my D&D campaign notebooks sit with my D&D stuff.
But I don't have a separate work/life/personal dev notebook. Shuffling them got annoying.