I have been to Korat several times. Some temples in town, more temples and mountains a bit away (would need car, difficult or impractical by public transport). With car and driver, I could spend weeks in Korat province. No gay nightlife that I know of. Few western tourists.
The train is a good idea, you can see construction sites of tunnels and elevated track for the high-speed train, very entertaining for me.
Khao Yai, unless you go to some specific temples or rocks or cliffs, was pretty boring for me. Just mountainous forest (I have no interest in the various wildlife that you can watch there).