When Puebla is Mexico’s Arequipa, Guanajuato must be its Cuzco or Potosi. Cuzco because it’s all built up against a mountain wall, a warren of little steep streets and unexpected vistas. And Potosi because of its silver mining connection of course. I stayed for two nights in the Valenciana neighbourhood (a splurge at the Casa Estrella de la Valenciana) and had the best views from my room and even better ones from the bathroom. The city lies in a “bowl” (somewhere I read it’s a ravine) between mountain tops, just like a smallish La Paz. I am throwing in all kinds of Peruvian and Bolivian comparisons, as Guanajuato resembles those countries more than the rest of Central Mexico which is very Spanish-colonial/westernized.
The prettiest building in town is the Teatro Juarez. Sorry, another comparison comes to mind: the Teatro Tomas Terry in Cienfuegos (Cuba), another opulent turn-of-the-century theatre. Teatro Juarez is open to the general public, and for a fee of 35 pesos for entrance + 30 pesos camera fee you can visit several of the rooms and of course the main stage hall. Its decoration is Moorish-influenced, very nice.
I finished my visit at the obscure Mummy Museum. It lies on the outskirts of town, probably outside of the borders of the WHS. It's quite a tourist attraction with over 100 "mummified" (dried out) corpses on display taken from the general cemetery when their families could not pay the grave taxes anymore. It's a bit of a gimmick. I did feel bad about it, think of your dead grandparents being displayed like this in their nightgowns.
Guanajuato also is a university city, which results in a young crowd and many cheap eateries & cafés. I did see less of a Mexican middle class here, which was so prominent in Mexico City and Queretaro - occupying the terraces of restaurants or drinking expensive coffees.