The Tourism Authority of Thailand lists 2 Khmer sites in the Province Ku Mahathat (Ku Ban Khwao) and Ptasat Ku Santarat, in Amphor Na Dun. In the Na dun area there are 2 more sites in close proximity to Ku Santarat. They are Prasat Ku Nang Khao and Ku Noi. There is another site at Baan Bua Mat in Amplor Borabue. There is 1 more site that I have seen in photos, but have not visited to confirm location, Prasat Ku Sanam in Amphor Wapi Pathum, which I hope to get to before Khao Pansaa.
You can see the locations of the Prasat by clicking on the album and viewing them in their Picasa web album
and you can see them in my Mahasarakham Google Map
View Mahasarakham in a larger map
There is 1 more ignored tourist site in Amphor Na Dun whichI will discuss when I post the photos of the new sites.
Once again these are all sites not listed by the Tourism Authority of Thailand