Get a hotel and get some sleep.

Thats some very bad flight timing's though!! There is really nothing much you can do at that time. Maybe go out and eat some local food and walk around Sukhomvit, but even then not much will be open you will probably end up being harrased by the go-go girls. Now if you are into that type of stuff then the sky's the limit. Very risky though... i wouldn't do anything there.