I spent 2 months in Cuernavaca in 2004 as part of a Spanish program from my university. I stayed with a host family and attended Chuanuac School ( I'm sure that is spelled wrong! ) It was a wonderful experience. If you are visit I suggest you go to the Palacio Cortez in the center of town.
While in Mexico I also went to the Shrine of Guadalupe, the silver mines in Taxco, the Pyramids at Teothuacan as well as the Ballet Folklorico in Mexico City. I highly recommend all of these activities.
Also when you are in Cuernavaca make sure you make time to go to the fiuca a.k.a. the black market. There you can fine wonderful things for a very cheap price. Extremely good looking knockoffs of things as well :) It is rather hard to find but worth the search. Just make sure you don't need to use the bathroom as it is dirty and filled with millions of dead cockroaches!
If you go to Cuernavaca let me know how your trip was. Enjoy!!