Journalling and other ways of keeping track of things that have happened
A calendar tells you what should happen in the future. A journal or diary tells you what you did in the past. You can combine them, and treat past calendar items as a sort of journal, but a calendar tends not to be a good format for writing down what you did or thought in any detail.
The main benefit of a journal is to have a place to record your thoughts, and those actions that you may need to remember in the future. The level of detail you use depends on your circumstances, time, and energy. For example, if you do scientific research, you'll have a lab journal or similar book in which you write down all your experiments and conclusions and thoughts related to them.
I use a set of text files (markdown files) for capturing the thought processes when developing software (they're actually part of a private Ikiwiki instance). I think out loud by writing down my thinking process, and try to do this with the minimum of self-censorship. The journal is not meant for others to read, and this gives me a lot of freedom to be as stupid and wrong and silly and opinionated (and occasionally rude) as possible. (Even so, I've published parts of my journal for the amusement of the Internet.)
Journalling can require a bit of effort, but it can be very helpful. The process of expressing thought processes in writing makes them more explicit, and often more clear. Anything that's hard to express tends to be unclear in the head, too. Later, a journal can be helpful to answer things like "what was I thinking?".
Apart from effort, journalling requires time. I type reasonably fast, so the extra time to write down my thoughts in some detail, as prose that's easily readable later, is not a big overhead. Others might prefer to keep a handwritten journal, and only jot down the more important bits, and stick to keywords. Or record audio, perhaps. Experiment with what works for you; maybe you'll find something, or maybe you'll find it's all wasteful.
The act of journalling should be as easy as possible. There should be minimal steps required to start writing, and the only requirement for the actual text should be that it's expressing what you're thinking as you write it. A plain text file works wonderfully; a simple word processing document would work too, if you prefer that kind of writing tool.
Apart from journalling, you may want to have some automatic logging of events in your life. Anything that you want to keep track of, if it can be logged automatically, is easy. Anything that requires manual effort is likely to not work quite so well.