We stop over in beijing capital airport, and my lay over is 21 hours.

Dear Vivian. Your time is enough to tour the Great Wall and the Mutianyu Section is recommended to you. It takes around an hour for a single trip to the wall by taxi and it costs CNY200-250. At the wall, it is suggested to spend 3-5 hours.

7 days great wall is open.. Until what time at night the wall open to the public and the taxi is all over the area..

Operating hours: from March 16 to November 14: 07:00 - 18:00 and from November 15 to next March 15: 07:30 - 17:00.

Generally, taxi service is available till late at night in that area, so you can easily take one after visiting. Or directly hire one for the round trip from the airport. Besides, many private cars are also available nearby. Negotiate with the driver for an exact price and get back to the airport. It is also safe enough.