Beijing 6 hours

It is not suggested to see the Great Wall, because the time is not available.
Generally speaking, you can take the local subway line 2 to reach Guloudajie Station where you can change to line 8 to reach Olympic Sports Center to see the Water Cube and the Bird Nest. Then you can take the line 8 to reach Guloudajie Station to change to line 2 to reach Jianguomen Station where you can change to line 1 to reach Tiananmen East. Then you can get out of the station to visit the Forbidden City.