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 and analyze 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 or smaller dotgrid notebook. Regular grid rule is OK too. These days I prefer writing with an F hardness 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.