Writing
Essays, notes after reading, citations worth keeping and questions I am still working through. Some pieces are finished. Some are deliberately not.
The cost of building the wrong thing quickly
Speed only helps once you know what you are building. Before that, it just gets you to the wrong place sooner.
Writing software you can still read next year
Most code is read far more often than it is written, usually by someone who has forgotten writing it. That someone is normally you.