Sounds like you are coming on on UA and departing on TG to BKK. I did this about two months ago, and asked the same question. The link should take you to some of the responses I got. The short answer is there is stuff to do if you are mildly adventurous and willing to figure out the trains (which I am told is not hard).

I was unable to sleep on the flight over and too lagged to be bothered. I wound hanging around the airport, which I do not recommend.

http://www.flyertalk.com/forum/showt...&highlight=kix