The night life in Beijing is very, very vibrant. Most people, particularly international students/foreigners go to San Li Tun 三里屯 District where there is a plethora of clubs and bars. You’ll meet people from every continent bar Antarctica.
During weekends, the city is packed with tourists from all over the country. You have to remember that there are over 1 billion people in China and Beijing is filled with so much history. Luckily, you’ll have your PKU student ID which will help you out a lot – you’ll be treated with a lot of respect and privilege. If you’re an American citizen, that will add even more bonus points.
For me and my friends, we mostly had class from 1-5pm on the weekdays. We would often wake up at 5am to explore the county, come back for classes, and go out again into the evening. There was so much to see that even after going out almost every day and night, there were places of interest I never got to see.
You’ll have a great time. I’m glad you’re going!