Learning to play an instrument is probably not something most people can get an 80/20 sort of benefit from, but it belongs in a class of activities in which some/many people can put minor effort in to reap the significant benefit of becoming a more interesting person, depending on one's innate proclivities. Other examples may include dancing, singing, drawing/painting, certain sports/physical activities, craft-work, etc.
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?