You can learn the basics of fitness and a healthy diet in a week. These are things that will improve your life in every way (mentally, physically, and financially). However, it is one of those things that is easier to learn than to apply.
If you really put your mind to it you could probably learn the japanese kana. It took me a couple months of like half an hour a day a practice to learn them but if you have a pretty good memory and trudge along you could do it. It makes reading Japanese computer manuals much, much easier, but I suppose the real value depends on the person.<p>Cyrillic and Hangul also come to mind as relatively easy writing systems to learn though I never really learned Cyrillic so it might be harder.
How to be fast on your computer. Remapping keyboards and being able to call custom macros/scripts. I use Karabiner for that.<p><a href="https://wiki.nikitavoloboev.xyz/macos/macos-apps/karabiner" rel="nofollow">https://wiki.nikitavoloboev.xyz/macos/macos-apps/karabiner</a>