Basically I did the same visit as Els did, so I will not repeat most of it. However I took a different transport approach and had a different experience on site. That´s what I will describe below. 
 
I chose also the Huai Khai Khaeng wildlife sanctuary as Els´ review was very helpful and also on the website thailnationalparks.com you get good information on many of the countries parks including this one. I wanted to visit the site without rental car (self-drive) so I chose the closest city to the headquarters where the visitor area can be found. The website has a detailed map of the different trails and watch towers available there. My decision was the city of Nakhon Sawan which I reached after a 5 hour train ride from Bangkok- Bang Sue Junction station (46 THB). From the train station outside the 100k people city there are small pick-ups transporting passengers into the city center (12 THB). I stayed in the P A Hotel (450 THB per night) and the reception helped me arrange a taxi for the next day for a full day trip. First she was very pessimistic that anyone would drive that far, but then she found someone. The cost was at 2200 THB for the full day. It took us 2 hours to get to the headquarters (15.608595138194318, 99.320538667944) by car. Because we told our driver that we want to go to the memorial for the environmentalist Sueb Nakasatian he drove past the first control point that Els described at kilometer 5 of the unpaved road. I think normally one needs to pay here but as he drove past we didn´t pay anything for the whole day. At kilometer 14 one meets another control point and our driver drove down through the river bed and up to the visitor center and memorial site. The river was quite dried out and I think with a normal car like we had only at this stage it´s possible to drive through here. Otherwise it would be a 15 minute walk from the control point to the visitor center, where there are some information on the life, death and foremost beliefs of Seub Nakasathian. 
 
At the memorial the guide Jeab approached us and we did the  Khao Hin Daeng Nature trail together, that Els had done as well. We saw a wild pig, many birds, many signs of elephants, many lizards under the leaves and insects. It was quite hard to take a picture of the animals as they were quite fast. It always felt like an elephant could appear and we would need to run. However it didn´t happen. After the 2 hour walk we were completely soaked from sweating. 
 
We also took the lunch break at the ranger restaurant as Els described. As the river was dry there was not much to watch. Interestingly our driver got a nice meal with rice from the cook, whereas we needed to eat our 7-11 snacks. Always better when you speak the language. In the end we even had to wait for the driver to finish his meal. Interestingly Jeab said goodbye to us and asked us and the driver to drive to the Peacock Watchtower. After reading Els review I did not believe that this would happen, as there were so many rangers around and I expected them to trace every step of ours. We tipped Jeab for the nice first tour in which he could name all the animals in English and was very observant in finding us animals. However his promise that at the watch tower an open gate and a ranger would expect us was not true. There was a closed gate at kilometer 8 of the unpaved road and neither us nor the driver who had obtained instructions from Jeab were sure if it´s the correct spot. In the end a ranger showed up and we walked from an abandoned house next to the road without guide (!) to the watchtower, where we obsevered 2 hours until 4.30 p.m.. There were quite a few birds around. Pigeons lived on the roof of the watch tower, we saw a few individual peacocks in the distance, some Red- wattled Lapwings in the dried out river bed and here and there a blue shining king fisher flying around. At the end a deer-like mammal showed up in the distance but disappeared as fast as it came. When we walked back to the car the driver had disappeared. We figured out that he drove to the ranger as he was nervous that we had diappeared because "it´s dangerous with elephant and tiger after 4 p.m.". We felt indeed a bit uncertain walking without ranger through the forest even though it was just a few hundred meters. The official opening time of the park and tower is until 4.30 p.m. though and we stayed within these limits. 
 
It feels like there is a lot of animal activity in this park however it's difficult to view it properly within the visiting possibilities of the park. May seems to be not a good period as due to the drought the big animals wouldn't come out to the water places. I enjoyed the remoteness and friendliness of this untouristy part of Thailand especially after experiencing the opposite a few days before when visiting the historical centre of Bangkok for a few days.