In the best of all possible worlds, you'd take two weeks in order to visit the whole region on a circular route. The good news is that even on a short trip you can get a sense of the region. If you are short on time, you may want to base yourself in Korat, a four-hour journey by train from Bangkok. The town can serve as a base for trips to the nearby Khmer ruins at Phimai. Surin, Buriram, and Si Saket, a little farther away, are also good bases from which to visit more ruins.