One great place in Mexico that hasn't been mentioned is Guanajuato. It's one of my favorite places to visit. Basically it's an old mining town that used to be the richest city in Mexico. It's filled with plenty of colonial style buildings, monuments, and historical sites. One of the things that makes this place unique is that it used to have canals very similar to Venice to prevent flooding. These canals are not needed and they have been converted to tunnels for streets. So basically a lot of the main streets are actually underground. The biggest (and darkest) tourist attraction there actually has to do with the soil in the surrounding area. The soil has some minerals that prevents decomposition of bodies. So the bodies become mummified. They actually have a museum filled with mummies and it is definitely a unique, disturbing, and creepy place to visit.
As for the more non-touristy stuff it has plenty of stuff going for it. It has a good university, plenty of theaters, good nightlife, and friendly people. It is also one of the safest towns in Mexico. Google some images of this town to see how beautiful it is.