Phnom Sosir Caves
On the road from Kampot to Kep, just before arriving in Kep, you'll see 3 small mountain a few hundred meters from the road. A sign saying "Phnom Sosir Resort" points to the left. Take the dirt road a few minutes, and you're there!
A Wat (Buddhist Temple) is at the base of the hills, and a short walk up the stairs takes you to another Wat. You can then go into some small caves in a few different directions. There's also a wonderful view of the countryside and the ocean from the top.
Worth an hour stop coming to or leaving Kep, Cambodia.
There's no charge, but you'll find local tour guides all over that will take a dollar or two to show you around.