Hey Claire,
Now is a great time of year to visit Japan! You’re going to have a wonderful time. The cherry blossoms will be blooming, so be sure to book all of your hotel/hostel/airbnbs in advance. This is the biggest piece of advice I have for Japan by far. When I went last year (around this same time) I stayed in the last room available in Kyoto.
Japan is a very friendly country, and you wont have any trouble with harassment or rudeness. One thing to be aware of is that unlike large parts of Europe, even in Tokyo most people (and by that I mean almost all people) do not speak English. I used a podcast series to learn some basic Japanese phrases as I flew over on the plane. I suggest you do the same. While not having any common language might seem like it would complicate your plans, most people will be so incredibly patient and helpful to you that it will hardly feel like a struggle. Though, reading signs and maps can be confusing. I suggest reading up on the general layouts and navigation tips for each city you plan to visit.
While there’s plenty to do in Tokyo, and you could easily spend several weeks there without seeing everything it has to offer, I suggest you at least make your way to Kyoto, or another less-huge Japanese city. Kyoto was my favorite. It’s considered the cultural capitol of Japan, and during cherry blossom season the whole city comes alive with visitors from around Japan, and the world.
A short list of fun things to do:
Go to an onsen spa- it’s a very cool cultural experience. Read up on what to expect, and how to behave, then go and relax in the burning hot waters of traditional bathing culture.
Go shopping- even if you only buy a hair clip, the Japanese retail experience is like nothing else I’ve ever known. There is so much care and detail put into the shopping experience that even in the most generic mall stores you will feel like an actual princess.
Stand in line for food- Seriously, if you see a bunch of people lined up in front of a restaurant, join them. They probably know something you don’t know and you’ll be happy when you find out what it is.
A short list of things to research:
Japanese traditions surrounding money and payment: Paying for things in Japan is very different from the way things are done in the US. Do your research on this aspect of the culture, so you don’t end up committing a faux pas, like handing someone money directly.
Train maps and city navigation: It’s easier to figure things out in the moment if you’re already familiar with the basic layout of the city.
Japanese Shrine Etiquette: You’re going to see a lot of shrines. It’s important to know how and what to do with them when you’re there.
Hope this helps! Have fun!