For a vacation less than 90 days, you only need a passport from your home country, and proof of your return flight in less than 90 days. Drugs were covered in another comment, but dont bring veggies or meat products into Japan, even if its like a Slim Jim thing you got at the airport, they’ll give you a hard time over it. The trip can be long, so its worth planning it out to give your future tired self less things to think about. Expect to be sleep deprived your first 2 days.
Travel agencies are good for booking hotels in advance, which i highly reccomend to do 1 month before. I stayed 2 weeks and that I think was a great amount of time. I booked the hotels and plane in advance, then had $2000 in savings to convert to cash yen for the actual trip. Use cash, Japan is very cash heavy. That was way more than enough spending money actually, i probably could have done it with half that amount. Fall time has great weather, and I avoided the more expensive spring season / cherry blossom bloom. That sounds nice, but tickets are more expensive that time of year.
Travelling in Japan, there’s the fast way and there’s the cheap way. For trains, they have the shinkansen and the regular regional trains, and for roads theres the side streets and the expressway. Beware expressway tolls. Getting out of Osaka wasnt too bad but getting back in beaned my wallet for ~$90 USD. Theres toll gates less than 5 minutes apart there. If you have 4 or more people in a car the expressway might be cheaper than the shinkansen, which you need individual tickets for each person. Using the expressway isn’t required, you can take side streets the whole way.
I had local friends who knew the language and could drive. They made my trip amazing. I tried to study some japanese language but when it really came time to interact with people i could not cut it. I really needed my friends to do anything more than sightsee and shop outside of tourist traps. My appearance and dress alone indicated me as an Obvious Foreigner and so people were quick to help and give me menus to point at. Even if your speaking skills were terrible, manners make up for it. Learn the local customs and formal etiquette, and people should forgive awful broken sentences and be patient with you. If you remember only one phrase: sumimasen. It’s excuse me / sorry / can i have your attention all rolled into one phrase.