One that serves as a “miscellaneous personal stuff” vault that helps me remember stuff, but that nobody but me would find useful unless you were stalking me or were trying to buy a gift for me. It has a page dedicated to stuff I remembered and spent an hour+ trying to find again, so that I can find it more easily in the future; a page for rating stuff I bought at the grocery store so I remember which ones I liked and which I didn’t; a page with the locations of this or that important document like my taxes; a page with the knitting patterns I’m working on and where I last left off; a page with the exact hex codes for my favorite color… stuff like that.
One for my creative projects: Dungeons & Dragons campaigns, notes for two games I want to make but may or may not actually bother to get off the ground…