An hour long eastward drive from Guanajuato would bring you to it's twin city, much longer to spell yet equal in terms of traits and features, San Miguel de Allende (read San Migel de Ayende; I'll use SMA henceforth). Queretaro airport is the nearest to SMA unlike Guanajuato which is served by Leon airport. This also has many buses plying from almost all important cities of Mexico, additionally it has frequent shuttle services from Guanajuato.
As mentioned before it matches Guanajuato in many aspects - architecture, weather, food but differs largely when it comes to crowd. San Miguel de Allende houses innumerable, especially old and retired, Americans and Canadians. Attributed to it's pleasant year all year round, many gringos have settled in San Miguel de Allende. As a consequence most people there speak English and one can find boards and menus in both Spanish and English. It's not just a touristic center but also a center for arts and theater. Instituo Allende, setup in 1950 is the flagship of art center and El Teatro Angela Peralta, an ancient theater holds many plays and festivals.
Apart from being home for art and theater, historically SMA is considered as the "Forge of National Independence". This was the place where rapidly growing revolutionary, El Grito, of Miguel Hidalgo received General Ignacio Allende as lieutenant and was greatly strengthened. Ignacio Allende is the local hero of SMA and his house has now turned into historical museum.
The city central square, located just opposite to the magnificent Parroquia cathedral, is where you find people with varied activity from noisy children to making-out couples to walking old men. The streets are cobblestoned like Guanajuato and are narrow, though the gradients are not as steep. There are classy restaurants around the square, the prices of which are just double of what you find in Guanajuato and are mostly crowded by gringos. I would recommend restaurants farther from central square for cheaper and more authentic food. I had best latte of my life in a place called "Cafe", a minute or two walk from the center. Since it's a place for artists, there are innumerable places selling their artworks and souvenirs.
Such a beautiful place has one thing in lacking - Parking space. Finding a parking space is like finding water in desert and heed my advice: Never park in any place without Parking sign. You're vehicle will be mercilessly towed by the Transit cops and getting back is the difficult, painful and slow.
San Miguel de Allende requires not more than four hours to traverse but the people are so interesting there that it's a heaven for candid shots. They are colorful and vivid and they don't get mad if you point the camera towards them. Though my stint with SMA was just about four hours, I would love to spend my life there capturing expressions of the people.