ds2 is right, -- the timing is such that you're not really going to be able to do anything. Flight lands (scheduled, that is) at 2120. Walk to Immigration can take 15+ minutes by itself, the airport is very spread out. Immigration wait can be anywhere from 0-60 minutes.

For my money, I'd just view it as an overnight stopover (which it is, of course). Take the Airport Rail Link to a nice hotel in the city (SGS is my Bangkok home, of course and right on both the Skytrain and underground lines), have a nice and relaxing night's sleep, good buffet breakfast in the morning and leave the hotel by 0945 to get back to Suvarnabhumi by 1030.