The park’s most-visited attraction is this revered cave sheltering a royal săh·lah (often spelt sala) built for Rama V in 1890. Scenes of this Khuha Kharuehat Pavilion bathed in streams of morning light (usually starting at about 10.30am) are widespread making this place famous across the kingdom. Even more interesting is the cave itself. The roof has collapsed in both of the large chambers allowing small forests to grow, adding an otherworldly ambience.
The trail to the cave is a 430m-long, steep and rocky stairway built into the hill. You'll almost certainly meet macaques and dusky langur on the way up. The path begins at picturesque Laem Sala Beach, flanked on three sides by limestone hills and shaded by casuarina trees. It can get busy here on weekends, but it's always peaceful at night and in the early morning.
There's no road access to the beach. Most people ride a boat (200B one-way) from the fishing village of Bang Pu. Note that these are wet landings, but in shallow water. Alternatively, you can follow the steep footpath from Bang Pu for a 1km hike to the beach. A boat ride to Laem Sala from the Hat Sam Roi Yot resort area costs 1200B and for an extra 200B they throw in a 'monkey island'.
A leisurely visit to Tham Phraya Nakhon will take at least three hours, but it could easily be extended into a full day with time on the beach, or even an overnight if you camp.