For day-to-day purposes, if you are used to Fahrenheit but not Celsius or vice versa, and all you want to do is get a rough sense of how warm or cold it is outside without having to do arithmetic involving fractions in your head, then remember that there are two temperatures in Celsius that are roughly the same in Fahrenheit but with their digits transposed: 16° C ~ 61° F, and 28° C ~ 82° F. You can then roughly interpolate/extrapolate by about 2° F for every 1° C.

The litany against fear

The mneumonic major system. Once I learned it by heart, it helped me memorize all kinds of numbers: cards, IDs, passwords, addresses…

Simple recipe formulas that are scalable

Anyone who isn't at least mildly interested that you know Morse code isn't someone you want to know :-)

Good filter technique.

A crowd pleasing karaoke song!

Subnets

Learn some alphabets of foreign languages. Russian is fun because some of the characters looks like English letters but have completely different sounds. Korean is also cool because it looks crazy complex but it's actually extremely simple.

A completely random ordering of a deck of cards. You can have a deck pre-stacked in this order, learn some false shuffles, have someone pick a card and place it back anywhere they want without marking its location in any way, and when you inspect the deck you know exactly what their card is. And they'll never guess that the way you did it was memorizing the order of every card in the deck.

I'm sure there are a lot more advanced ways to take advantage of this, just a handy ability to have in your back pocket (literally).

The Doomsday rule.

Not necessarily the part for calculating the day of the week for any arbitrary day centuries ago, that's just a useless party trick, but for the current year so you don't need to pull out your phone to check. Knowing that 1/3 (or 1/4 on a leap year), the last day of February, 3/14, 4/4, 5/9, 6/6, 7/11, 8/8, 9/5, 10/10, 11/7, and 12/12 are all the same day of the week, that this year they're all Tuesdays, and next year they're all Thursdays, is mostly easy to remember and very frequently useful.

How to pronounce names in different cultures.

The Ballad of Sam McGee.

Have you heard the tragedy of Darth Plagueis the Wise?

I have this thing about astronomy. Kind of a perspective thing of our place in the cosmos. I try to remember all the distances of planets from the sun and distances of moons from their planets. Also the diameters of solar objects. There's other factoids I try to remember about neighboring solar systems and galactic bodies. For example I remember the black hole at the center our galaxy is called Sagitarius A and its mass is 4M suns. The black hole at the center Andromeda our closest major galaxy at 2.5M light years is 25M suns. The black hole at the center M87, the closest active galaxy at 50M light years is 4B suns. I didn't look that stuff up so tell me if I didn't get it right.

If you drive, the 3-4-8 second tailgating rule

Recipe for cooking. Useful for having a good meal and make other people love you.

Powers of two, squares and cubes up to 20 and the NATO phonetic alphabet.

The metric system.

Thirty days has September, April, June, and November.