I have 2 and half days in BJ ie Fri to Sunday, what is the best things to cover in these 2.5day

You can use one day to visit the Mutianyu or Badaling Great and then go to see the Bird Nest and the Water Cube at night.
The second day can be spent in the Forbidden City and the Tianan Square. In the late afternoon, you can go to Houhai area to enjoy the the Hutong Culture.
On the third day, you cn go to the Temple of Heavens to have a look and then you can go to the Wangfujing area if you have time.