Si Thep is about 250 kilometers (3.30h) northeast of Bangkok, and a bit tricky to reach with public transportation. From the Mochit BTS Northern Bus Terminal there are regular express buses to the province of Phetchabun, taking Highway 21 where I got off the bus shortly before the intersection in Si Thep village. After just a few minutes I made a deal with a Tuk-Tuk driver who spoke no English, but with the help of some photos, he understood what I was looking for. It was a leisurely 15 minutes drive. There is an entrance fee of 100 Baht for foreigners (about 3 USD) and that includes the ride in an electro train from the small museum at the entrance to the main site. You need about 40 minutes to have a look around. Khao Khlang Nai, the central stupa was a Buddhist monastery decorated with stucco figures (dwarfs and various animals), in the Dvaravati style of art of the 9th century. Prang Si Thep is the most impressive monument in Khmer style, built with brick stones. Smaller stupas have collapsed and only their platforms remain. 
The return trip to Bangkok was even more challenging. The long distance buses did not stop in Si Thep, and I had the take the Tuk-Tuk to the next bigger town, about 30km away, Chai Badan, where they have a bus stand, located at Sura Narai 14 Alley, with hourly departures to the capital. However, it turned out to be a slow minibus, taking in some villagers on the way. Count to be about 8 hours on the road. A good idea might be to combine it with visits to Ayutthaya or Lopburi and stay overnight.