Kaosan: packpacers heaven with bars, clubs and discos, a good combination of foriegn and local crowds.
RCA: Over ten dance clubs (808, Route 66, etc), live music and chill out venues (Ratri Samosorn, etc) or even the Gentleman club like the Forte, located as a long street linking the Rama IV and Petchaburi road. It is where you'll see many Thai teenagers hang out especially on weekends.
Lower Sukhumvit: bars, pubs, dance club, A gogos, you name it, it's there! Mostly targeted towards international crowds.
Thonglor and Akekamai are also areas for night owls. These areas can be packed with most upscale locals and some japanese business man. The popular venues are such as Santika, Curve or gentleman club at Sherbet and etc.
Many hotel venues: There are also places to chill out and listen live Jazz music with recommended to ones such as the Living Room at Sheraton Grand Sukhumvit, the Dilomat Bar at the Conrad Bangkok or Bamboo Bar at the Oriental, or the 360 at the Millennium Hilton.
Seems like one day is not enough to explore all these interesting scenes in Bangkok. Even living in Bangkok, we can not keep track of all cool places in the city. So if you are here, just try things bits by bits. You'll just fall in love with the city for sure.