Beijing has some of the world's grandest architecture.  The "Forbidden City," started in 1420, is the world's largest palace complex and was home to 24 Ming and Qing Emperors.  The building names barely hint at their magnificance:  The Hall of Supreme Harmony, The Palace of Celestial Purity, The Palace of Terrestrial Tranquility, The Hall of Mental Cultivation, The Hall of Joyful Longevity, and so on for a total of over 9,000 rooms.  Enter via The Gate of Heavenly Peace and allow at least an hour to see everything ;-)

  Nearby are the Temple of Heaven complex and the Lama Temple (the largest Buddhist temple outside of Lhasa).  The Summer Palace is a few miles away, and the Ming Tombs and The Great Wall are a couple of hours drive north.