Phra Nang Beach is the showpiece beach at the southern end of the Railay peninsula, framed by cliffs, caves, and karst towers in the water. It is popular for a reason: the setting is almost absurdly scenic.
The beach gets crowded with day boats, so timing matters. Come early, swim before the midday rush, and walk to the cave shrine at the end of the sand if you want the full Phra Nang scene.