Khao Yai National Park, Thailand
Where: Khao Yai National Park is located near Pak Chong, approximately 4 hours by train from the capital Bangkok.
Money: 1 USD buys 30 Thai Baht (May 2013), park entrance fee of 400THB (USD 14)
Language: Thai
Incredible waterfalls, diverse wildlife and most of all, cool temperatures. (The lowest ranges of the park lie 350m above sea level) The park boasts a network of hiking trails, but some effort is needed to get there as this is not a typical tourist spot. Overcome that, though, and there are some amazing sights for the adventurous traveller, including the Heo Suwat waterfall, made famous by Leo DiCaprio (give that man an oscar!) in the Beach. Excellent information here.
Big, Big World 