Will you stay around Tiananmen Square? So, I think you should spend one day in visiting its surrounded spots: the Forbidden City, Beihai Park, Houhai bar street and Hutongs. If time allowed, you could go to the Temple of Heaven and Wangfujing Dajie.

As fo the Great Wall, it is suggested that you take a taxi to Deshengmen or subway to Jishuitan (close to Deshengmen). Here, you could take a bus to Badaling Great Wall directly. In the afternoon, you could go to visit the Bird's Nest and Water Cube.

Of course, if your time is urgent in Beijing, it will be helpful to participate in a tour group.

We just returned and spent one day seeing the Temple of Heaven and being picked up from our cruiseline. SEcond day was spent seeing Tiananmen square, Forbidden City, Summer Palace and some other little things. (but friends of ours did Temple of Heaven on the same day as these other three things which would be great if you can take a lot of walking in one day). Another day we went to the Great Wall of China and Ming Tombs. (a must do to the Great Wall). So in the two days - I would do the four big sites I listed above (worth getting a driver/car and having meals included) and second day do the Wall/Ming Tombs. By the way, we were there six full days and I feel the things I listed were the highlights. Hope this helps you.