You can take airport express train to Sanyuanqiao and then change to Line 10 to Bei Tu Cheng Station. At the station, you can make a transfer to Olympic Special Line to the Sport Center, Bird's Nest, Water Cube. Get back to Beitucheng and take Line 10 two stops back to Huixinxijie Nankou to catch Subway Line 5 to Dongdan. You can get out for a walk to Wangfujing, then further to tiananmen Square then west further to Xidan, where you can take airport bus directly back to the airport. Dongdan, wangfujing, Tiananmen, Xidan are all along the route of Subway Line 1. You'd better return back to the airport 3 hours ahead of the flight departure.