Everything:
Recipes.
Things to do
Birthdays, dates not to forget.
Useful addresses: plumbers, emergency numbers.
Important things about you computer, How to...
Books you intend to read.
Things you want to buy
Wish list.
Christmas card list, menus, presents you think of in August
Arrangements for things you organise: ie: quiz night, Leaver's Do, coffee morning, gluten-free meals for that friend.
Clever quotes.
I live by by notebooks, started when I was working,
Unfortunately, I did have eleven spiral bound unfinished ones from Paperchase, now reduced to five during lockdown