Khao Sok National Park and especially Chiew Lan Lake are one of my favorites in Thailand ! The most classic excursion here consists of taking a boat trip on the lake, visiting a cave, and possibly going for a trek in the jungle. But the must-do tour is the formula 2 days and 1 nght in a floating hotel on the lake. There are also many treks, kayaking, bivouacs in the jungle options. And if you're lucky and have a experimented guide, like I had, you may see wild elephants on the shores of the lake.