Originally Posted by jiejie

Usable daylight hours in late December are around 08:00-16:00.

Tuesday Plan: Forbidden City, followed by Houhai/Shichahai hutong exploration. Duck dinner. Do this on your own, by taxi/subway/foot.

Wednesday Plan: Hire the driver for full day service hotel door to airport door. Have breakfast and check out of hotel around 08:00. Temple of Heaven 08:30-10:30 (assuming you are in a central hotel not one on the north/northeast side). Drive to Mutianyu arrive around 12:00. Have lunch first in Mutianyu village. Up on Wall 13:00-15:00. That is about all you can likely stand in winter, especially if the wind is up--it's quite cold and exposed. Depart by 15:30, to airport 16:30. I presume you are taking the Air China nonstop to REP. This is more than enough time before an 18:45 flight departure.