Making notes.
Having friends. Helping each other live better.
Regular exercise. Also, listening to music while exercising.
Buying useful things. (As opposed to noticing that you don't have them, but doing nothing about it.)
I wonder whether "mirror their body language" has an analogy in written text. Something like "write like a GPT trained on the other person's texts". Or rather, an artificial intelligence could decide the topic of a message, and then write/say it using words (and sequences of words) from the person's vocabulary.
The very unexpectedly high utility subject for me was hypnosis. You have to learn and know very little in order to do some amazing things.
'If you want someone to like you just mirror their body language' is as easy to implement as it is to read, solves a common problem, and does it exceptionally well.
Are there other subjects which are similarly surprisingly powerful and implementable?