Khao Yai National Park
We spent pretty much an entire day getting from Ko Si Chang to Khao Yai. Ferry back to Si Racha, motor-rickshaw back to the main road, local bus to Chonburi, big bus to Nakhon Rat Chasima (Khorat), bus to Pak Chung, then pickup from our hotel. This was aimed at the backpacker market and had the nicest rooms of the trip but (relatively) expensive food.
The following day we did a tour of the national park, which included a short hike. We saw gibbons (in the distance), a wild elephant (strolling along the road with tour vehicles following behind and reversing in front of him), greater and wreathed hornbills, a giant black squirrel, pig-tailed macaques, and deer, among other animals. We also saw the scats or signs of black bear, wild pigs, elephants, etc. and a range of trees.