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?
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.)