Khao Sok National Park is a nature reserve in southern Thailand containing dense virgin jungle, tower-like limestone karst formations and the artificial Cheow Lan Lake. It is home to rare species such as the giant parasitic Rafflesia flower, hornbill birds, gibbons and tigers. This park can be explored by elephant-back safari, hiking trail, and raft, canoe or kayak via the Sok river.
Khao Sok National Park is definitely worth going to, especially if you are going to be in Southern Thailand anyway. It is easy to access from popular destinations like Phuket and Krabi, and also is not too far from Koh Samui (and its neighboring islands), being in the same province.