Other than clubs, there are the sidewalk pop-up bars & restaurants around Suk & Asoke & Soi 11. On Soi 22 in Queens Park Plaza some bars in the back will stay open 'till the sun comes up if you're spending $$. There's also a Bar on Soi Cowboy called Penny Black that stays open at least 'till 4. It's on the L facing Asoke, down by Country Rd. Make sure you go to the 3rd floor as the 2nd floor bar is gay. It fills up with SC girls with no customers that night. Up on Soi 33 I've crawled out of Barfly & Lookie Lookie at 6am a few Xs too. There's probably some good recent threads on this on Pattaya Addicts & Bangkok Addicts. I haven't been to any of these places after hrs, post-coup. BTW, you may be charged 700B Airport Tax when you get back to BKK. IME, it's been hit or miss.

I'd head right to Nana and get a seat at Big Dogs or some such. Place that looks onto the street. Watch the zoo a while, then head around the corner to one of the sidewalk bars. Soi 38 has good food but might be closed by 3am. Dunno, club might be it for really late, but for me an hour or two watching Nana then zip up to Soi 38 for an hour of eats and chillin there would do for me. Then cab it back to the airport around 4 or so. If yer layover was any longer I'd be thinking eat, sleep, wake up head to airport. Might be better if yer in on a weekend, fri or sat night.

There are enough inexpensive and expensive hotels near the airport. Maybe a 100-150 baht taxi ride away. You could get some sleep and maybe a massage. If it was a long flight from the US that might help your feet.
Get back to the airport early for a breakfast in the foodcourt at the lower level.

2nd thought: Depending on your options, leave out the hotel and spent the savings on the massage.