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Daily notes. I have a template that prompts me to fill out a number details I might otherwise forget.
A wiki of people that helps me remember details about people I meet or have worked with. Makes it much easier to keep in touch and to remember important dates in their lives.
Sortable todo lists, with due date and urgency information. I can add to the lists directly from any other note using a Dataview formula with the Tasks extension.
Career plans. Project plans. Gardening plans. Recipes (there’s an awesome extension that imports recipes from the web).
Any random writing I might want to do, from short stories to rough drafts of letters to stream of consciousness mind spew that I want to review later.
I use the Auto Note Mover and Dataview extensions, along with backlinks and tags, to keep all of my notes organized automatically. I use the Linter extension to make sure things are formatted nicely. When I started using Obsidian, I used the Importer extension yo easily pull in all of my existing notes and lists from Evernote and Google Keep.
Honestly, that barely scratches the surface.
This. So much this. Every time I start a new project I’m so glad to have these notes to refer back to.