Bangkok has a reputation of being a wild city a night. The city at night has loads of different options for all tastes and wallets. Ranging from cheap bars along the sidewalk to big ultramodern clubs to red light districts to extravagant roof top bars high up in the air.
The large 'backpacker hang around' of Khaosan is where most foreigners who are traveling on a shoestring end up. Here are many bars and outlets to choose from. If you don't feel like drinking, you can always to like the younger thai crowed an just walk around looking at the 'colourful' foreigners.
Chidlom is the place to be if you want to spend some money and want a bit more class. There are no palaces here that gives your spouse grounds for divorce, just by you entering the place.
Here you can mix with the IN-crowd and expats at Hi-class restaurants, Jazz clubs and Wine bars.
Many of the younger thai population hang out at the Ratchadapisek area and at the RCA (Royal City Avenue). A long bar street that host large bars/clubs. Be aware, it gets really pack, especially on weekends.
In and around the Siam Square are you will find many different things to do, all close by. Such as bowling, cinema, shopping and cafe´s.
Or why not try to go on a dinner cruise on the Chao Praya river. You pay for a dinner plate and sit down and eat as long as the boat takes you about on the river and it´s sights.