It took me five years to finally make it to Mount Koya, although I lived in Japan at the time. Maybe because it is quite a train ride (or better said several train rides) to get there, but actually the journey itself is already worth the trip.
It is the religious center of Shingon Buddhism in Japan and you can actually feel it throughout the whole valley. I highly recommend to stay at a temple overnight, visit the Okunoin, Japan's largest graveyard, and if you like hiking, try one of the hiking trails that go around the center of Koyasan, offering beautiful views from above.
