Mount Koya, while it may be a trek to get there (only because of multiple transportation changes - train, train, funicular, bus - not because of the time to get there), is a spiritual haven and the heart of Shingon Buddhism in Japan. Founded by the revered monk Kobo Daishi in the 9th century, this temple town offers a unique experience for visitors seeking tranquility and cultural immersion. What you'll want to do here is a temple stay (see the link I provided), with many temple lodgings to choose from, all seeming fairly similar. From staying overnight in these traditional accommodations to exploring ancient temples and hiking pilgrimage trails, Koyasan promises a peaceful retreat from the hustle and bustle of modern life, and everything is walkable when you get there. As a spiritual hideaway that remains notably not very touristy, it's a 10/10 recommended experience. UNESCO site too.