Also known as the Black Valley, this site is both gorgeous and mysterious. It has lush vegetation and waterfalls, and air that is misty and clean; the silence inside is almost deafening.
It's very easy to get to, even by public bus, although arranging the trip through your hotel is easiest. Take your own food and drinks, as the options there aren't great. The area is best avoided on Sundays and public holidays, when it gets swarmed by crowds.