Take California Boulevard to the north end of town, then make a hard right (just a bit north of a certain unmentionable national fast food chain with golden arches) and take the overpass over the freeway. When you come to the T on the far side of the overpass, turn left and go a couple of blocks; the restaurant is on the right.
El Sombrero is a popular place, nicely decorated, definitely the place to take people who want Mexican food in a refined and pleasant setting. It has grown in several steps from a fairly small building to the current building. Their last remodeling jag enclosed the rear patio (yay, says I, remembering some rather diligent mosquitoes that used to infest this pleasant spot) with a high ceiling and clerestory windows, and an indoor fountain (if you don't mind smokers; they have nonsmoking rooms indoors, though).
The menu is fairly long and there are some unusual dishes on it, such as Enchiladas del Mar, made with shrimp and crab. Although I often complain about excessive salt, the food is generally pretty good. The one sensory defect I notice most often is that the green chile they add to the dinner plates has rather too much juice in it, so it tends to make the rice soggy.
My typical order is #8, the stacked enchilada plate, with blue corn tortillas and chicken.
They have Chile Beer (from Arizona, with a small red pod in every bottle).
Service is generally decent, although some of their servers are not so great. Not a place to go if you're in a hurry. They take plastic.