Here's a place you go for the ambience, not the food:

http://www.bedsupperclub.com

Quite simply the most jaw-droppingly decorated club on the planet. Reservations obligatory (book at least a day or two in advance) and a single serving daily, the course starts at 9 PM but you have to show up before 8.30. The food is a daily changing surprise menu, generic but very well executed international cuisine with a few Thai touches. Weekends are B1200/person for 4 courses, weekdays B1000/person for 3 courses, floor show included but drinks and service excluded.

For a nightcap, head to Vertigo / Moon Bar atop the Banyan Tree on Sathorn Rd. The drinks are overpriced and service isn't that great, but the absolutely stunning views of Bangkok are worth it. The place is aptly named, so don't go here if you have a fear of heights -- the only thing between you and a 60-floor drop to the street is a metal railing...