For next time -
attractions we didn't see
Most obviously, we didn't visit the Great Wall. That's a whole day trip and a subject unto itself. The other long day trip from Beijing is to the Ming Tombs, 50km to the northwest.
Inside Beijing, attractions we would have liked to have seen but didn't manage to fit in included the Ancient Observatory, the Bell and Drum towers, Fragrant Hills Park, the Old Summer Palace, and Beihai Park. There are also plenty more museums and art galleries.
You'd need a week in Beijing just to see the highest profile attractions.
We used the Lonely Planet Beijing City Guide, which was fine. But there's no shortage of travel guides to Beijing!
I hope to return to Beijing.