Roughly halfway between Khorat and Pak Chong, little is known about the ancient city of Muang Sema (โบราณสถานเมืองเสมา), though it was clearly important in its day. It began as a Dvaravati outpost in the 7th or 8th century BC, during which it is presumed to have been the capital city of a small kingdom. Later, probably in the 10th century, it was occupied by the Khmer, although Mahayana Buddhism continued alongside Hinduism. Many artefacts found here are on display at the Phimai National Museum.
Other than one evocative reclining Buddha, it's mostly ruined foundations and a pair of moats. While Muang Sema isn't spectacular, what you get here is a sense of being off the beaten track. Beyond the temple, there's no tourist infrastructure other than a few explanatory signs, and you'll most likely have the place to yourself.