Getting there
The Nankai railway runs from Namba down to Gokurakubashi, where a cable car climbs the final stretch up the mountain into Koyasan itself — a journey of roughly two hours door to door, and one of the more scenic train rides available as a day trip from the city.
Okunoin cemetery
Okunoin, Japan's largest cemetery, stretches for over a kilometre through old-growth cedar forest, its stone lanterns and moss-covered memorials leading to the mausoleum of Kobo Daishi, founder of Shingon Buddhism, who Koyasan was built around in the 9th century. It's atmospheric enough that many visitors rate it above any single temple building on the mountain.
Why an overnight beats a day trip
Koyasan is unusually well set up for overnight stays in shukubo — temple lodgings that include a vegetarian monastic breakfast and dinner, and the option to attend a morning fire ritual. Given the travel time involved, many visitors find a single overnight turns a rushed day trip into the most memorable stop of a Kansai itinerary.
Koyasan sits at real altitude and runs noticeably cooler than Osaka year-round — worth an extra layer even in summer.