Although a flight attendant advised me against it for safety concern, I ventured off for a stroll through the area behind the Camino Real, across from Mexico City's airport when I had an unexpected overnight before an evening flight (Apr 2007).

The 2 hours were a wonderful frolic through Mexican culture, with friendly natives and a sensory-overloading street market.

Here are some pics in a slideshow format if you're interested:
Pics from neighborhood behind MEX's Camino Real

I didn't encounter any safety problems. YMMV. Maybe more suitable for a 4-5 hour layover than a 9-hour, though.