eli5

I have, what I hope, an unusual and possibly beneficial question for anyone here and, hopefully, I can get as many different answers as possible.

I was wondering, what interesting technology, language or tool can I learn in a week or so by reading a book and/or practicing. Something that has a potential to greatly improve my productivity or maybe just my understanding of how things work, or maybe even my life. Anything.

To give you a number of subjects I'm already familiar with, so you don't suggest them: Rails, Javascript, html/css (you can imagine the stack), also quite good with Linux and its tools, using Vim.