arrive in beijing at midnight from sydney with 12 hour layover on way to london. what can i do?

Well, you are suggested to find a hotel nearby Sanlitun Bar Street. There are many bars and clubs after the midnight. Or you can go to Donghuamen Night Market and Longfu Temple Night Maket. There are many local snacks for you to choose. In the next early morning, you can visit Tiananmen Square and see the Flag-Raising Ceremony. You can alao see Monument to People's Heroes, Great Hall of the People and Memorial Hall of Chairman Mao. In addition, you can see the Forbidden City from outside.
Notably, don't forget the time. You need to go back to the airport two hours in advance.