If you have only…
One day in Beijing
First, visit Forbidden City, then Summer Palace. You will have to be in the Forbidden City just after the gates are open, and do you sightseeing a little bit faster than otherwise. These attractions are big and require quite of walking. Alternatively, you can take a tour in one of the many tour agencies and visit Forbidden City and The Great Wall of China.
If you feel that either of these two options would be too much for you, then you can do your only day in Beijing as described in Day 1.
Three days in Beijing
On the first two days do Two days in Beijing, as described above. On the third day do Day 3.