Use of the 24 hr visa free can take tour of Great Wall, Beijing

I am arriving to Tianjing port of Beijing by cruise ship. I plan to take a half day tour to the Great Wall before going to Beijing airport to flying out the same day. My cruise doesn't go to any Chinese port before ends at Tianjing port. I know that I can stay up to 24 hr without entry permit but I don't know if I can go from the ship to the Great Wall and then go to airport to take my flight in the evening OR I have to go directly from the port to the airport and stay at the airport until my flight.
If somebody knows the answer please advise .
Thank you very much.
Lien