I keep a bullet journal to help keep track of my goals and organize my thoughts. It uses the Arvelie calendar to help me manage my time more effectively.
For each 14-day month of the Arvelie calendar, I write a heading for that month and the goals I intend to focus on and complete for that interval. I leave space next to the goals so that I can write notes after the fact regarding why they didn't get completed, what went well, how my intentions changed, etc.
Every day (or almost every day), I write freeform notes which can include ramblings, progress reports, designs, pseudocode, poetry, among other things.
My preferred daily journal is a 5x8 dotgrid notebook. These days I prefer writing with an F-sharpness drawing pencil. I find the graphite doesn't smudge and the point stays sharp for long periods of time; I don't use an eraser most of the time, but, to me, pencil feels better in the hand than pen regardless.