I've been there for many times because it's my favourite beach in Thailand. Tong Nai Pan beach is far away from the pier and hide away behide big mountains. A small village at the beach are so nice and peacful. Many tourist don't know many beautiful beaches of koh Pha Ngan so they are have just the experience of full moon party at Rin beach.
I had been stay at many beaches of Pha Ngan island but my favourite are Thong Nai Pan Yai and Thong Nai Pan Noi beaches. At there you can walk to the waterfalls from the beaches and enjoy in the jungle when you want to move from beach for a while. More than the beautiful place is nice local people who always welcome people from over the wolrd.
Pha Ngan island is the only one island in Thailand where were changed not much as another islands.
Email to me if you want to know more about the island.