This is great sight if you're interested in astronomy, science, ancient technology, the meeting of East and West, or you just need a break from the noise and crowds. The observatory was built in the Yuan Dynasty (1239-1368 AD). It was put to great use by the Qing Dynasty, and Jesuit missionary, who built most of the astronomical devices on the grounds.
There are tons of great English captions throughout the museum. Tickets cost 10RMB.
The observatory can be a bit hard to find; however, if you walk directly across the street from Jianguomen subway station, exit C, you'll find it.