Japan is still very far away from being a cashless society, so the first thing to worry about is money.
Exchanging currency doesn't yield best rates. Currently , the one with the best rate/fee is 7-Bank. an IC card can technically be used as a debit car in many tores, vending machines and restaurants throughout Japan.
Get a JR Pass
If you are planning to travel around the country , a 2 or 3-weeks JR Pass can save you time and money. It's super convinitent and will give you unlimited access to (Some types of)Shinkansen and JR-Branded trains, buses , and ferries.
We will include this to your plan if needed.
Hold on to your garbage
Japan is a SUPER clean country. Thus it's counter-intuitive that such impeccable cities have so few trash cans.
When locals are walking, they don't smoke or eat so there's no need to for bins. In any case if you find yourself with the odd piece of paper or plastic, the quickest way to get rid of it is inside a convenience store.
Currently, to avoid "COVID" Japanese wear masks on the streets, stores, restaurants...etc
At the moment it would be better to have a mask carried with you.
Do NOT tip
This one is important.
Do not - under any circumstances - tip in Japan.
It is considered rude.
No matter how great the food or exceptionally good the customer service is.
CAN'T smoke everywhere
Like in other places in Asia, smoking on the streets is forbidden. When you are outside, make sure you go to a designated smoking area, usually next to big buildings, shops or train stations. Smoking outside these areas is extremely disrespectful and one of the biggest offenses you can do in Japan
Tattoos are not welcomed.
Across Japan, tattoos are strongly associated with criminal gangs like the Yakuza. Although this is allegedly changing with increasing tourism, foreigners with tattoos are not particularly welcomed by some.
You can expect to be denied entry in some places, especially onsen(hot springs), saunas and pools.
Japanese people are extremely polite. Which is one of the foundations of their society and bowing is the way to show respect when you meet someone. But this gets complex when you realize there are different types of bows depending on who you are getting with. Howevery, if you don't feel confident to contorol this bowing technique, don't worry. Japanese people are understanding and most likely you'll get some chuckles.
Japan is a safe place
Japan is one of the safest countries in the world, where people go out of their way to avoid any kind of conflict or upset. It's one of those places you ca leave your laptop or phone on the table when you are going to the WC.