Anything you might use. Think of yourself as an artist before the era of the portable camera. If you don’t sketch something now you’ll forget it. So with words and ways of expressing what you see, feel, do and think. Get those ideas and descriptions down as soon as you can.
Try to work them up that evening, that day. Within a couple of days. Think of this as nurturing a seedling that will have to be transplanted in a day, week, even in a year or five years time. It’ll wait for you, but only if you have made it robust.
Write, and rewrite, toy with it and layer it. Have you recreated a sense of the moment as it was experienced the first time? Maybe. This effort to recall the moment creates a multitude of connections in your brain, some logical, many not, some becoming fixed, some floating, all transformed every nanosecond more that you live. There is no stability in it, not on the page and never in your head.
Then use these ideas somewhere. In a short story, or in a character or context description. It’ll come in time. It’ll become easier.
No automatic recording device can do this for you. A gadget works in absolutes, in numbers. What it takes for you is fixed. Write it down.
Like many of us, I'm sure, I wordpress and go online. I conceive and store ideas on these things. None yet gets close to offering the emotional power of what I said, and where I said it forty years ago: in a school kid diary, in a letter to my grandfather, even typed up with the portable typewriter I got one Christmas. Boxed up and stored I know have a way back into the child's head, that young teenager's hopes and observations. I even printed out filed and boxed stuff from the Amstrad in the 1980s and the various MACs I had in the 1990s and that originated on the Psion in 2000. Anything I thought would be safe on a floppy disc, or Zip drive is probably lost. I can't figure out to read them on a modern device. A box full of paper is another thing.
At 53 my life has been short. At 89 my father-in-law is going through his 'archive' - an academic and educator his house has over the years become a physical expression of the contents of his brain. There are books three columns deep to the ceiling, there are bedrooms stacked with boxes of papers and newspapers. As most of his faculties fail he now has a PhD student at his side to give some order to his archive, and to his life. Sharing it with one, on paper, at this stage, has to be more rewarding and effective than doing it digitally in a blog with wiki-like affordances.