As I have already posted TAT lists 2 Khmer ruins in khonkaen. The 1st being Prasat Peuai Noi, which I have posted before and you can see again. You can also see the location if you open the photos in Picasa Web. These are the geotagged photos of my visit yesterday to the 2d Prasat. It is not road signed, but not out of the way. If you are into khmer ruins as I am it should be on your list. Someone said that the modern building next to the site was a museum. I saw no signage in thai or english to confirm that. The diagram in both thai and english is legible and useful in describing the area.