Khao Ngu (meaning Snake Mountain in Thai) has long been popular with Thais as well as hundreds of chubby monkeys, which you can feed bananas or corn for five baht a pop.
Buddhist artwork in one of the caves, Phra Phutthachai Tham Reussii, dates back to the seventh century and includes an inscription in Sanskrit, "the merit of the hermit with Gupta". On our last visit the caves were locked, so it was just us... and the monkeys.
About 400 metres off the main road you'll come to a statue of a cobra protecting another of the mountains. Energetic visitors can climb up a series of 446 steps to find a small Buddhist pavilion. It's really quite a stunning view from the top, and you can easily see back to Ratchaburi and temples and farmhouses dotting the countryside.
Keep going and you'll find more caves that look rather rundown. These contain various ancient stone Buddha images, often in pieces piled on the floor among chip packets and Coke cans. If you get bored looking at the caves, sit down somewhere and watch the frustrated local mongrels chasing the swarms of monkeys around. The monkeys have a good time, even if the dogs do not.
You can get there by taking a yellow songthaew from Rotfai Road. Jump out when you see a gold-painted Buddha statue carved into the side of one of the mountains. It can also be combined with a visit to Khao Chongpran, which is around 10 kilometres further, though you will need chartered transport to do this.