If you want to travel in winter, the place you should go first is a drug store.
Winter in Japan is beautiful, but it's cold, especialy for sightseeing. We have to wear warm clothisng, such as sweater, coat and so on. Even with such clothing, we still often feel cold outside. There is a prerequisite for such a cold winter of Japan. That is the disposable pocket warmer, which is called "tsukaisute kairo" in Japanese.
We can buy them at a drug store or a convenience store. The price is from about JPY300 to JPY600 for 10 packs.
There is a pocket-size bag inside. Being exposed to air, It starts to get warm little by littte. The average temperature is 50℃, and the warmth of over 40℃ lasts 12 hours to 20 hours. The 2nd picture's one is standard type. There is also attachable type that we can stick on clothing.
You can stick it on your back, socks and other places you want to stick it.
But be sure to stick it on clothing, not directly on your skin, and take it off when you go to bed.You can get low temperature burn.
The standard type is called "haranai taipu(type)" and the attachable type is "haru taipu" in Japanese. The small one of the 1st pucture is "kutsusita yo" or for socks.
Before going for sightseeng, go to a drug store or a convenience store and ask a shopstaff," tsukaisute kairo no haru/haranai taipu wa arimasuka?" which means " I want to buy a standard/ attachable type disposable pocket warmer.".
With tukaisute kairo, your winter travel in Japan must be more comfortable!