At a minimum, in my opinion, Mexico City is worth at least a three day visit. Look at possible itineraries here on Frommers website and also go online and search for Hemisphere Magazine's "Three Perfect Days in Mexico City" article to get more ideas.
Also read through the "sticky" posts on this bulletin board.
Take the same precautions you would in visiting any North American city, stay in the tourist zones, and you should be just fine during your visit to Mexico City.
While on the Pacific coast in Ixtapa it's not a good idea to go outside of the greater Ixtapa-Zihuatanejo area. The cities are located in the Mexican state of Guerrero which unfortunately has seen more than its share of narco-violence. I wouldn't worry about it at all in Ixtapa-Zihuatanejo but if you have any ideas about renting a car and going off exploring on your own outside the cities I would strongly advise against that.