arriving in beijing 9:00 am, leaving at 8:30 pm.

Yes. I think you can only choose one attraction to visit during your layover in Beijing. If you would like to go to the Great Wall in the suburbs, it takes at least four hours to get there and back on the road and you also need to set aside at least two hours to check in for the onward flight. And the Forbidden City is closer to the airport than the great wall and you can take the airport line and transfer to subway to Forbidden City.