Those are the most popular options:

Old Beijing at night:

A. Tian'anmen Square, National Grand Theater and Changan Avenue

B. Forbidden City watchtower

C. Beihai Park (enter)

D. Jinshan Park (enter)

E. Qianmen Street and Dashilan old area (enter)

F. Houhai Lake Hutong area, Silver Ingot Bridge, Skewed Tobacco Pouch Street  (enter)

G. Drum Tower and Bell Tower

H. Nanluoguxiang Hutong (enter)

New Beijing at night:

I. CBD area (The Place, China Zun Tower, CCTV Tower, China World Trade Center Tower III, Galaxy SOHO etc)

J. Taikoo Sanlitun (enter)

K. Wangfujing street (enter)

L. Bird Nest and Water Cube