Reddit has a great thread about skills you can learn in just one week, particularly with the help of resources like YouTube. Some of the skills below do require materials, but most of them don’t!
Lockpicking. “It’s one of those skills that takes a moment to learn but a lifetime to perfect. Think of it like cooking – just because you can make a mean grilled cheese or scramble some eggs doesn’t mean you’re ready to take over for Gordon Ramsay. But it’s definitely a satisfying thing to learn.” – Reddit user
How to sew. Chances are, you might already have a sewing kit at home, but never quite mastered the most useful techniques required to mend a tear or hem the length of a garment. “Good skill to have for simple clothes repairs, can extend the life of many favorite garments by years.” – Creative_Recover
Raising an eyebrow. “So, I did it by holding down one eyebrow with my hands rather hard and raising the other. That way, you know how it feels and what muscles you have to use. Do it a little every day. You’ll find that soon enough, you can do it without the hand, and bam!” – GeneralSilverini
Knitting. “I picked it up yesterday and I already made a washcloth in less than 6h.” – sneezypeasyqueezy
Getting better at cocktails. “Do you have a pint glass? Or any glass that holds a pint? Do you have a spoon? A normal, adult spoon? Do you have a knife? Do you have booze? Do you have ice? Congratulations, you have a bartending kit. Cocktailing is like most other hobbies: it can be done with $15 worth of tools or with $1500 worth of tools, and largely the quality of the results will have more to do with the skill of the person rather than the expense of the tools.” – CowboyLaw