Language.
I've learnt several physical stuffs (e.g. bodybuilding, music), several emotional stuffs (e.g. meditating, music) and other intellectual skills (e.g. game theory, the humanities and programming of course...) and if I had to choose one to recommend to the HN community, it's language.
Unfortunately, it's high reward [0] also comes with high effort. Even with the best resources, it will take time and patience. But you can start at anytime of your life and at anywhere.