I am from Houston, but have lost track of the number of times I have been to San Antonio. It is one of my favorite Texas cities. Be sure to visit one of the local Mexican restaurants while you are in town, meander down the Riverwalk, and soak in the atmosphere.
The Radical Reference group has set up an informational wiki on San Antonio for Midwinter. It contains many good suggestions on getting around, eating, and leisure time.
The Convention and Visitor’s Bureau of San Antonio also has a nice site with a variety of suggestions for out-of-towners.
If you have time, I suggest taking the trolley to El Mercado, where you can shop and eat in a Mexican style market square.