San Miguel de Allende, an hour and a half east by bus, is the most popular day trip — a comparably picturesque colonial town with a stronger expat-American presence and a different feel. Dolores Hidalgo, an hour further on, is where Father Hidalgo gave the Grito of Independence in 1810 and where Mexico's pottery industry has its capital.
The silver-mining sites of La Valenciana are technically still in Guanajuato city but feel like a day trip — combine the mine, the church, and lunch in the village.
Cristo Rey del Cubilete and the colonial mining town of León (an hour west) are also reachable as half-day trips by local bus.