The most well-known part of this WHS - Koyasan - I visited on my second trip to Japan. This temple town has the usual Japanese abundance of souvenir shops and restaurants: don't expect it to be very serene.
Most of the temples are grouped in the western part of town. Their designs are all very different. Okuno-In, the temple that houses the body of Kobo-Daishi, is on the other side of town. Going there you have to cross a large cemetery, covered in moss and hidden under huge dark trees.
An experience not to be missed here is staying overnight in one of the temples. The local Tourist Information can arrange this. The Fudo-In was my accommodation: very quiet (only 2 young monks seemed to be living there), with great food and a fine traditional bath.