I chose the nearby city of Lam Narai based on various factors including Thomas' review with transportation options as a base to explore Si Thep. It has a "Grand Hotel" (600 THB per night), a train station that is a 2 hour direct ride away from Ayutthaya and some other city advantages like restaurants, transport etc.. It turned out that a car rental with taxi driver would only work our at 2500 THB for a full day which was too much for us as we paid the same for the two national park visits in the west of Thailand which involved much more driving (around 200 km vs. 60 km to Si Thep and back). However our hotel reception arranged us a Tuk Tuk for 1000 THB for 5 hours for the next day which worked perfectly fine.  
 
We arrived as only tourists around 9 a.m. at the Khao Klang Nok site which however had many vendors waiting for tourist customers. A friendly guard of the site provided us with chairs and turned on a TV which presented us the English video about the WHS and the inscription criteria etc.. That was a good introduction. Otherwise on site one can only see the pyramid kind of builduing which was recovered from plants and soil around 20 years ago. After around 45 minutes we moved on with the Tuk Tuk to the main site- the ancient village.  
 
There we needed to pay 100 THB entrance per person and jumped on one of the electric buses that brought us first to an excavation site with a human skeleton and an elephant skull. Here our brown leaflet was heöpful that gave us some information in English. Afterwards we were dropped off at the three main monuments of the old town that are all in walking distance of one another. Each of them comes with an English text sign. I would say 30-60 minutes are enough to see them in detail. You can pick any of the small buses going back or walk about 15 minutes. Close to the entrance there is an exhibition hall with English texts on Si Thep that I found really well written and that gave a timeline of what happened in the area and detailed and reflected desciptions on the purpose of the three components. The text must have been written shortly before the inscription at the end of last year.  
 
It took us 45 minutes to drive back to Lam Narai from where we took the morning train to Kaeng Khoi junction (1,5 hours) and from there to Pak Chong (1 hour after 1,5 hours waiting) in order to visit Khao Yai NP (also WHS). Alternatively you can take a bus from Lam Narai to Saraburi and from there another bus to Pak Chong. I prefer train rides though. In the evening of our visit day to Si Thep a heavy rain and thunderstorm occured in Lam Narai. Probably the start of raining season after many super hot and dry weeks in Thailand. 
 
I asked and researched a bit on how to visit the third component-  Thamorat cave at the top of Thamorat mountain. It's a buddhist shrine and about 15 km from the ruins. In march every year there is a religious ceremony on climbing the mountain to the cave. The guard told me that I would needcto ask for permission to hike to the cave at the Si Thep center, which I didn't do in the end as I was not planning to do it. It's a 5 hour tour to go up and down according to my research.  
 
The ruins are from an older period than the ones I saw a few days before in Ayutthaya. However much less remains of them. Nevertheless the stuccos and human and animal carvings are quite nice and really old. When visiting the national museum in Bangkok our Suisse guide who lives in Bangkok for many years told us about a French scientist that doubts the strong connection between Si Thep and the Dvaravati culture that seems to be the storyline of the Thai inscription of the site. Due to this opinion he was disallowed to continue researching on the topic and proofs and materials were kept away from him. I don't know what's the truth on that. I I found it an interesting however not overwhelming site that gives a rough idea of what was going on in Thailands' present territory around 1000 years ago.