Hand warmers

Not what you asked for (sorry) and frankly I feel like this is a really environmentally unfriendly rec, BUT when I was in Japan in February (and it was snowing and absolutely frigid) I would buy these pretty much daily at convenience stores and they truly were the key to staying toasty warm. I have also once or twice bought them in bulk to use as little heating pads for headaches, and they’ve come in clutch for that as well.