I write down one line (about 80 characters) about what things I did each day. Originally I intended to write down "accomplishments" in order to incentivise myself into being more accomplished, but it has since morphed into also being a record of notable things that happened, and a lot of free-form whining over how bad certain days are. It's kindof nice to be able to go back and figure out when exactly something in the past happens, or generally reminisce about what was going on some years ago.