Check out this video for a recommendation of what to do in Guanajuto if you only have 24 hours. It is put together by an expat who lived in Guanajuato for about a year and contains a nice mix of touristy things and restaurant/bar recommendations with a local touch. Take some or all of the suggestions, but watching the video will give you a good idea of where you might be able to start your Guanajuato adventure!
We have found this site very useful for giving down-to-earth recommendations and evaluations of site-seeing options in Guanajuato. It also provides a lot of the history of the places you might see, which is a plus. Prices listed on the website are in pesos.
Isreal comes highly recommended by Talia's parents as a versatile, very interesting English-speaking tour guide who's up for anything from a walking street tour to a day trip to a tequila factory. City tours range about $10 USD/person for 3 hrs. Israel also does airport pick-ups and other excursions. Prices are moderate; email him for more info if interested in engaging his services.