Skippy: am mulling over doing the same thing on the way back from South America, though I may end up having as little as 24 hours there.

I'm told that the ruins of Teotihuacan (30 miles NW of the city centre) are worth heading for:

http://www.mexperience.com/guide/arc...eotihuacan.htm

If you find out any more about things to do when time is short, would appreciate if you post them.