Maybe orders of magnitude of different things, e.g. weights, volume, population counts, distances, timescales etc. Makes Fermi estimates much easier if you can recall a comparable fact that you know the magnitude of

The NATO alphabet. This will make spelling unusual things over the phone so much easier.

Multiplication of medium sized powers of 2

If you excel at mental arithmetic, I'd suggest committing the values of standard distributions to memory. To begin with:
P(-1 < X < 1) is approximately 68%
P(-2 < X < 2) is around 95%
P(-3 < X < 3) is nearly 99.8%
Naturally, you can also familiarize yourself with other common distributions. I've found this handy for swiftly assessing the validity of arguments that include numerical data.

the URL of "Never gonna give you up"

10-20 important phone numbers

The 100 most common words in the most common languages

The amendments of the constitutions?

A few keyboard shortcuts of programs you use daily, especially for actions you use often.

i'm surprised nobody mentioned ACX's essay "Memorize A Few Basic Numbers To Have Intuitions About Other Things"

all the greek letters upper and lower case -> engilsh name and back. this way if you need to read and say math, you can actually talk to others and have an internal monologue

Basic writing/number systems of major languages e.g. Cyrillic script (Russian), Arabic script, Devanagari script (Hindi), Hangul script (Korean), hiragana/katakana script (Japanese). It's surprising how many place names, prices, cognates, and loan words you'll find yourself immediately noticing. Also it breaks down one of the psychological barriers to starting to learn languages using those scripts. Plus they look cool 😎

Emergency services phone numbers of different countries, including obscure ones, like poison helplines

Multiple types of knots (clove hitch, becket bend, figure eight, bowline, etc)

Converting Imperial to Metric

Converting Pounds to Kilos

Righty Tighty, Lefty Loosey

↑ ↑ ↓ ↓ ← → ← → B A Select Start

Countries/capitals of world, order of US presidents, and major foreign leaders

Olympics. There are only like 60 of them, yhey are regularly spaced, and they tie into some other stuff.