My husband and I have stayed at Quinta Las Acacias in Guanajuato! It is not in the town center, but it is a beautiful hotel and offers every amenity. The staff was very friendly and the breakfasts were great!

Here is a link to Mexican Boutique Hotels. http://www.mexicoboutiquehotels.com/lasacacias/

However, since you are only staying one night, you may want to stay in the center of town. Guanajuato is a beautiful city and the weather is beautiful all year long. Hope you have a good time.

Happy travels, Sharon and Bob Houston, Texas