The hook
On a mountaintop wrapped in cedar forest sits a thousand-year-old monastic town, and at the far end of its graveyard a monk has been sitting in meditation, his followers believe, for twelve hundred years.
01Why it's worth it
Kōbō Daishi, the monk who brought Shingon Buddhism to Japan, founded a monastery up here on Mount Kōya in the year 816, and it grew into a town of temples, more than a hundred of them, eight hundred meters up. The heart of it is Okunoin, Japan's largest cemetery: a two-kilometer path through towering old cedars past more than two hundred thousand graves, moss-covered and leaning, to the mausoleum where Kōbō Daishi is said not to have died but to sit in eternal meditation, still brought meals twice a day. You can stay the night in one of the temples, eat the monks' vegetarian food, and wake for the dawn prayers. Few places in Japan feel older or quieter.
02How to experience it
You reach it by train and a cable car up the mountain, a couple of hours from Osaka or Kyoto. The real way to do it is to stay overnight in a shukubo, a temple lodging, so you have the town after the day-trippers leave. Walk Okunoin at dusk or after dark, when the lanterns are lit and the cedars close in. Bring warm clothes; it's high and cool even in summer.
From the locals
Kōyasan turns up in the r/travel "Japan off the tourist trail" thread as a flat recommendation with no hedging attached: "Koya-san is amazing — it is a mountain area filled with Buddhist temples and monasteries."
The brevity is fair. What people come back talking about is not a sight but a night: staying in a shukubō, a temple lodging, eating the vegetarian monastic food, and going to the pre-dawn service. The same commenter's wider advice for the country applies especially here — set your bags down for two or three nights rather than day-tripping, because the mountain empties out completely after the last cable car and that is when it is worth being there.