Visit Khao Yai National Park (Nakhon Ratchasima)

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).

Admission fee: 400 THB per adult and 200 THB per child.
The park is open all year from 6 am to 6 pm.
From the Northern entrance (Pak Chong), songthaews can take you to the Heo Suwat Waterfall which is more than 20 km away for 800 THB (8 people max).
You can camp in the park (I have not tested but the best camp site seems to be Lam Ta Khong) or rent a bungalow. The Visitor's center is about fifteen kilometers from the Northern entrance (Pak Chong). It has a restaurant and there you can book your accommodation (from 800 THB), tents (250 THB + a hundred bahts for cushions mattress and sleeping bag) and night safaris. There are two departure times for the night safaris, 7 pm and 8 pm (500 THB for 1 hour in a pickup of 6 people). You can also go through a local agency or via the official website of Thai National Parks to book tents and bungalows but the website is not very intuitive and payment can only be made by bank transfer which may lead to high additional fees.