A wall that just goes around part of the city Xian, which was formally the capital. Small enough, circumference, that I cycled around it in an afternoon, with an ice cream as the reward after bumping around the cobbles on the bike.
There is feng shui museum on part of the wall. Interesting metal signs and buildings to peer over the wall at and decorative building along the wall. Probably did not need to ride the whole wall to see the sights but perversely it was good fun!