Can you recommend a place to visit for one day in Beijing?

Forbidden City(https://www.travelchinaguide.com/cityguides/beijing/forbidden.htm) is worth your visiting. You can take the airport shuttle bus line 2 to its termal at Xidan, just 1km away from the Forbidden City. Here, you could take subway line 1 for one stop to the Tiananmen West, namely the Forbidden City. You can visit the Tian'anmen Square by the way.
Well. something need to mention, since your flight is at 10:00pm, you should back at the airport around 8:00pm. You may need to leave from the downtown city at about 5pm to the airport in case the traffic jam. If you arrive at Beijing airport in the afternoon, it is not advisable for you to go to see the Forbidden City.