Peg system – it's a very simple rhyming mnemonic that helps you memorize a list of 10–20 items by quickly assigning visual imagery to each number. It's more of a parlor trick than anything, but it's pretty fun.

Here's the list for ten (but you can go much farther than this)

  1 - Sun
  2 - Shoe
  3 - Tree
  4 - Gore
  5 - Hive
  6 - Sticks
  7 - Heaven
  8 - Gate
  9 - Vine
  10 - Hen

Then you take the list of things (grocery list, etc) you want to quickly memorize:

  1 - Viking
  2 - Ink Pot
  3 - Matador
  ...

and quickly create tiny visuals for each one:

1 - A longboat rows around the spherical face of the sun each of vikings dipping their flagons into the solar flares

2 - A old man accidentally knocks over his ink pot spilling it onto his shoes

3 - An Tree ent holds a cape and taunts the uruk-hai into charging at him

You'll find its trivial for somebody to call out a number and to recall the associated item. Over time you'll also find yourself getting extremely quick at it as well.

The algorithm I mentioned in the OP is here: https://en.wikipedia.org/wiki/Doomsday_rule

I practiced the algorithm by replacing my doomscrolling time on a trainer app (https://whattheday.com/) over the last 3 weeks. I went from taking 2-20 minutes to usually get the wrong result to about 95% accuracy in 10 seconds.

This training app to teach you to play piano by ear and to sight read also got posted today: https://news.ycombinator.com/item?id=46556210

1. Rubiks cube

2. Wordle. It’s much more a game of probability than anything else so start by eliminating the most common consonants and vowels. I am averaging word selection on the third row.

3. As a typical software developer can get a better job by memorizing all the details of common digital transmission mechanisms from the specifications. It’s how I escaped JavaScript insanity.

4. You can get a gardening green thumb by memorizing plant details, knowing the chemistry of your dirt, and buying old bananas by the case for conversion into a fertilizer slurry.