Haeinsa temple, founded in 802, is my favorite among those I visited in Korea. It is in the mountain of Gayasan park and we have to walk a bit before getting there but the place, which leads to meditation, is particularly well chosen.
This is one of the three jewel temples in Korea with Tongdosa and Songgwangsa.
In Buddhism, the three Jewels are the Buddha, the Dharma (Law) and the Sangha (community). Tondosa temple has relics of the Buddha Sakyamuni (sari) and is thus the jewel temple of Buddha. Songgwangsa temple was an important Buddhist temple and so it is the jewel temple of the Sangha. As far as Haeinsa is concerned, it has the Tripitaka also called the Three Baskets, which are the Buddhist sacred texts (more than 80,000 wooden plates). This version of the sacred texts is the most complete (it would take years to read everything), and is classified by Unesco. The birch planks were soaked 3 years in sea water, boiled, and then dived again in sea water. Wood was thus protected from rot.
There are several other temples around Haeinsa temple.
And you can get more of Haeinsa with a "temple stay". You will sleep in the temple and wake up at 3 a.m. to meditate with the monks. Reservations can be made with the Korea tourism office.
The temple of Haeinsa is away from large cities and if you come from afar, and you want to stay a bit in the evening, you will have to find an accommodation. And fortunately there are many yeogwans (kind of cheap hotel) near the bus station. There will probably be people who will show you as soon as you get off the bus. It was an opportunity for me to stay in a yeogwan different from those that one can find in big cities. We slept on the floor on a duvet called ibul (the equivalent of Japanese futon). Generally the heating is underfloor, and it is called ondol.