Guanajuato has some of the most interesting historical and architectural sites of many places in Mexico. The colonial buildings, small winding streets, and pedestrian only areas provide great entertainment and are truly sights to behold. It is considered a World Heritage Site for good reason with some of the churches dating to the mid-1700s and with several excellent museums and other attractions.
Some warnings- First, the downtown area is small and if you are staying anywhere out of the central district you are in for a lot of walking up and down some very steep hills. Regular travel maps do not show elevations, except perhaps as a number but think of it as walking up and down Lombard Street in San Francisco. If you do this more than a couple of times you are in for a good workout!
The city has a huge universty student population and tends to be very noise, even into the late hours of the night. Be prepared to hear a lot of singing, noise, etc. specially on the weekend.
Finally, the food is not the best I've had. We ate at four different restaurants and all but one had food I would consider only as 'passable' at best. The prices at a decent sit-down restaurant are in the $12- $15 per plate range. Being mexico one would expect better food but the city and state are not known for their cuisine.
We did find an excellent restaurant called Le Parisien (review upcoming as soon as Trip Advisor accepts it as a new listing) which served a fixed price meal for 50 pesos (about $4,50 US) that is excellent. They have both French and Mexican plates, it is well decorated and the service is very good. It is located about a half block from the Diego Rivera museum.