There are a few things to consider/take note of when using the restrooms in Japan, both in public spots and in your own hotel. There are a few things that make them a little bit different than their Western counterparts.
Space age toilets – A vast majority of toilets in Japan are very high tech, especially when compared to Western versions. From heated seats, automatic lids, and control panels that control nearly everything (some even have white noise) they are incredible pieces of technology (which is crazy just for a bathroom). You will figure them out quite quickly but that first time the seat raises up for you due to the motion sensor, it’s an incredible thing.
Have a squat – You are in for a treat if you have never seen or heard of a traditional Japanese toilet, better known as the squat toilet. Not to be too lewd, but it’s essentially a hole in the ground and it takes some getting used to. I found one while exploring Meiji Shrine, so even in the middle of Tokyo, these old traditional toilets are around. They are something else so make sure to read up on these old school ones.
Baths – Baths are common in Japan which means finding a standard Western shower is kind of hard to do in most hotels. Most bathrooms in modern Japanese hotels are designed for baths but do have a shower head that can be used for a traditional western shower, but if you are staying in traditional Ryokan and other older hotels (or away from the bigger cities), you may be taking a bath. Usually (for traditional baths), the entire room is designed for the bath with a tub, a place to sit, and a bucket to fill and pour over yourself (if you want to). It’s part of the culture so be prepared for it, especially if you head to an Onsen, it’s how they operate.
What paper? – Paper towels are hard to find in public restrooms in Japan. It makes sense, especially in cities like Tokyo with so many people, the amount of trash would be hard to keep up with and keeping the bathrooms stocked would be even harder. This does make it hard to dry your hands or if you need a towel so make sure to carry a handkerchief with you, it is a useful tool to have with you when you need to dry your hands off.
Cramped – A large majority of hotel bathrooms are small and often very cramped. After staying in multiple hotels across Tokyo, Osaka, and Kyoto, I learned very quickly that space is limited in bathrooms so be prepared for that. You won’t have much space between the bathtub and sink and if you are expecting a counter for all of your toiletries, be prepared to be disappointed. These aren’t huge things to worry about but they surprise you when you first step into your hotel room.