Xi'an's old city wall stretches about 14 kilometers around the city and has been carefully restored. Built to help defend the city, and wide enough to allow four carriages to travel along it side by side, it's been kept pretty much empty except for the nicely restored corners and four gates. That means you can hire a bike and cycle along a really surreal, empty stretch of the wall—with a bustling modern city on one side and the old city down on the other. I cannot recommend this enough: It's one of the coolest things I've done on vacation in a while. My favorite part was when the sun set and the lanterns and wall were lit up for the night. It was absolutely beautiful.
The South Gate is the main area for hiring bikes, and stays open until 10 p.m. But you can drop the bike off at any gate if 14 kilometers is too much. There are also tandem bikes and big motorized carts that can take you around faster.