If you have only one day in Beijing, I would like to suggest you visit:

Tiananmen Square, Forbidden City and the Mutianyu Great Wall



Visit the Tiananmen Square, stroll around there for about half an hour then enter the Forbidden City.

In the afternoon, visit the Great Wall at  Mutianyu section.

The Great Wall is about 70 kilometers from the city center. I can help you rent a car or van for the day.