exercise, food, meditation/planning/metacognition training, some positive psychology tricks, social interaction, clothing and personal presentation, playing an instrument. There are probably lots of others.
Playing an instrument
Really?
strongly disagree with "learning an instrument". I wasted lots of time on that. The costs are high, and all the benefits are either dubious or can be more cheaply obtained through other means.
At least for classical instruments, getting good enough to not embarrass yourself is quite expensive, both monetarily (equipment, teachers) and in terms of time. At >1k hours of practice and ~$10k invested, I'm just barely good enough to get paying gigs (which I'd lose in a second if the people paying me knew anyone who'...
This is the question asked by John Cook on Twitter. He lists responses from different people:
Mine are: quantum mechanics, Python, cooking, the language of philosophy.
What learning curve do you wish you'd climbed sooner? Give reasons and stories if you feel like it. Do you think other people should climb the same curves?