We've done the full day tour in Khao Yai National Park and stayed overnight in the park after the tour. The second day we did the trail number 1 close to the visitor center and afterwards another half day tour by our recommandable operator Greenleaf outside the national park which however was the most memorable visit.  
 
For the full day tour we were picked up early in the morning by a Greenleaf driver at our hostel in Pak Chong which is connected by a train line to Ayutthaya and Bangkok. The car brought us to the operators' place on the road towards the most popular entrance of the huge Khao Yai NP. This area of the road is still full of commercial life incl. 7-Eleven, Hotels and restaurants. With a pick-up with benches we drove into the park. The price of 1500 THB is decent regarding the fact that it includes the 400 THB entrance to the park and enough food during the day. After spending some time at a nice viewpoint we had about 30 minutes at the visitor centre where we could arrange a bungslow for the night at the cost of 560 THB with fan and electricity. It's 1,5 km from the visitor center. We were prepared for going in a tent, but were happy that we could arrange the bungalow. There is a website to book the places online but it's so bad that it's impossible to use it. On the weekends and vacations the park gets full of Thai tourists and accomodation prices rise 30%. Probably it will be hard to book a bungalow spontaneously and the campsite is 6.5 km away from the visitor center with no food sources between 5 p.m. and 7 a.m. and in another campsite there died a tourist some time ago by an elephant stepping on him at night while sleeping in the tent. So for us the camping was more an emergency solution.  
 
After the visitor center, that offers a nice snd short exhibition on activities and animals in the park, we walked trail 3 with our guide. First it goes through open territory towards an observation tower that gives good views on a mineral lick. Later it goes into the forest. We saw hornbills in the tree in the distance, some nice figs that have grown impressively big and many interesting insects and spiders. The highlight was a thin, green whip snake sleeping on a branch next to the trail.  
 
After lunch we visited the beautiful but not super impressive Haew Su Wat Waterfall. It felt like a great place to have a nap or meditate we felt after a long day. Finally we searched for elephants but just by driving around on the road. We didn't see elephants but apparently the do run around on the streets sometimes in the evening. We saw some deer and stags though.  
 
After refreshing in our bungalow we were picked up by another pick up from the park rangers after sunset for the "night safari". Basically it was the same like searching for the elephants during the tour just in the darkness. We had booked the tour including the pick- up at the visitor center. It costs 600 THB per car and we found some others to share the tour with. Compared to Thailand so far it was quite cool on the open vehicle so one could even consider wearing a thin jacket. The guide searched with a huge torch in the forest for animals. We saw a fox, some deer and some porcupines which was the highlight. Interpreting the guides behaviour porcupines are really easy to find as the driver didn't even really stop for them. For me it was the first time seeing them though so that was very cool. The tour takes 1 hour.  
 
We were super exhausted after the day and slept long into the next day so that few time remained. We used it for the nice interpretation path starting at the visitor center. The first car passing by had kindly picked us up on the road to bring us to the center. On the path we saw many lizards and insects. The signs were interdsting when readable. The visitor center has a map with all trails in the area. They also arranged us a private transfer back to the Greenleaf office for 800 THB. To Pak Cjhong they charge 1000 THB and to the entrance gate of the park 500 THB. From there public pick-up shuttles bring you back to Pak Chong for a low price. 
 
The halfday evening tour from Greenleaf doesn't go into the park again but close to it and was the actual highlight of our visit. After a refreshing bath at Ban Ta Chang spring we drove to the Wat Sa Nam Sai buddhist monastery in which we visited a bat cave with many bats and a lot of poo. At sunset we were below a hill next to another bat cave where we witnessed how 2 million bats went out of the cave to go hunting. The sounds and images are simply impressive. It's somewhere in the countryside next to the amphoe nong khwang. After the tour, that cost us 300 THB per person, we were driven back to Pak Chong by Greenleaf.  
 
The Khao Yai National Park is a good place to enjoy the Thailand nature. It has decent infrastructure for visiting, comparably pleasant climate and all the animals and forest types to offer. Visiting by your own car is probably easiest however visiting by a tour is also a nice way to explore incl. an English speaking guide. Visiting on your own is a bit difficult and also only 3 trails are possible to walk besides the roads without guide. Additionally I would not like to meet the elephant without vehicle. I must say that from the short visits I had I could not really tell the difference between the three Thailand natural WHS as they seem to have similar fauna and also flora seems comparable. However as the Khao Yai was the easiest to visit and the best one to dive deeper into the matter, I rate it a bit better than the Huai Kha Khaeng wildlife sancruary. Kuri Buri NP I liked most as we saw the wild elephants and gaur but I guess it would have been possible to see them as well in the other two WHS.