8 days is plenty of time for Tokyo and its surrounding. Many people usually start their Japan travel rabbit hole from Tokyo. Big city, lots of tourist attractions, very good mass transit system including subway, trains, buses etc. But make sure to avoid public transit during rush hour, unless you want to experience getting packed like sardines. Be sure to set a day aside to check out Mt. Fuji, but check the forecast before picking the day, it gets blocked by clouds a lot. There is not very good public transit to get to Mt. Fuji, or rather, it takes a long time if you go by public transit. But you can hire a ride to get you there.

If you want to see history, culture, temples, shrines, and Japanese castles, go to the Kansai Region, which has several famous cities / prefectures close by, including Osaka, Kobe, Kyoto, Nara. Rich in history because Kyoto used to be the nation capital of Japan, way back when. If you do end up going to Kansai, be sure to check out the Kansai Thru Pass, as it's a several day all you can ride public transit pass for tourists only. You need to show your passport to buy one, but it's a pretty good deal if you plan on taking public transit throughout the area.

Then there are some rather popular but not as transit friendly places, such as Okinawa (way down south, sunshine and beaches) and Hokkaido (way up north, probably still frozen and full of snow in March). They are both nice places to visit with lots of nature and natural scenery, but best if you rent a car or hire a tour guide / driver.

You may want to check this out so that you can admire cherry blossoms 🌸 in late March in Japan: https://livejapan.com/en/article-a0001033/