Stretching over 4 provinces, Nakhon Ratchasima, Nakhon Nayok, Prachinburi and Saraburi, the Khao Yai National Park covers an area of ??2,168 km². It is the third largest national park in Thailand and listed as UNESCO World Heritage site, it is also the best known in the country. Its proximity to Bangkok (1h30 to 2h drive) and its jungles, mountains, meadows, waterfalls, rivers and especially its wildlife including wild elephants, also make it the most visited. People come for nature, waterfalls, including the famous Heo Suwat Waterfall shooting location of the movie
The Beach, but also and especially to observe the animals. In addition to wild elephants that you only cross if you're lucky, you'll find gibbons, hundreds of bird species, deer and deer, monkeys, epic pigs, snakes, etc... impossible to list them all. The Visistor's center organizes
night safaris in pickup of 6 people that allow you to see some animals but generally speaking it is a lot better to visit Khao Yai with an agency or a local guide. By yourself, you will only fly over things knowing that distances are big so you will need a transportation otherwise your visit of the park will be very very limited. Many guesthouses and hotels in the area offer 1 or 2 days tours, I recommend
Greenleaf, I have excellent feedbacks about them but there are others.
There are several trail courses in the park but most of them require a guide (it is imposed by the park) and anyway it is better to not venture alone in the jungle. Note that the nmber of visitors very significantly increases on weekends and holidays when many people come from Bangkok for a quick nature getaway. The most popular and crowded period being Songkran (mid-April).