There are a lot of places to go and things to do in evening in Beijing.
If you are looking for a modern night pub, Cocobababa will be a good choice as well as countless bars in San Litun area and Houhai area.
For a Beijing flavor arrangement, the link below could be your reference. Do you think it's cool driving a motorcycle sidcar in the ancient Beijing central city in the evening?
http://www.templeside.com/tour/tour.html about 6 years ago
I prefer Houhai to Sanlitun on the basis that it's more scenic - but that's irrelevant once you find an atmospheric place to settle down for the evening.
Don't forget that a visit to the Tiananmen area is wholly different after dark. You can't go into the square itself but can wander around the 4 sides. about 6 years ago