Sanlitun (三里屯) this is the center of nightlife in Beijing, located beside the embassy area in Chaoyang district, it comprises a main "bar street" divided into north and south sections, a side street with more casual (and cheaper) bars, and several large clubs/discotheque at the north gate of the worker's stadium near by. Sanlitun has near legendary status amongst travelers, but you are just as likely to be irritated by pushy bar-owners or DVD sellers as you are to be charmed by its bars.

Hou Hai (后海) is a man-made lake surrounded with trendy restaurants and bars in the central part of Beijing. A great place for a beer, and also to watch local Beijingers (of all ages) enjoying themselves.
Houhai is very fashionable and trendy in contrast to the seedy Sanlitun Bar district. There are many different bars and restaurants that line the shores of the Houhai Lake. The place is interestingly decorated and there is good music with a small dance floor. The area is a good place for people watching and taking a stroll. When we generally talk about Houhai, we often mean Shichahai which is composed of three lakes - Qianhai Lake, Houhai Lake and Xihai Lake. All the three lakes are connected and lined with lots of bars and restaurants. A boat house with boats you can rent to paddle along the lakes.
Add: Houhai lake, Lotus lane, north of Tiananmen Square and the Forbidden City.