I don't know specifically about San Antonio, but if you are planning on making a trip through Austin we have a brewery called 512 (its our area code) and they make a wonderful IPA, its my favorite Texas beer. They are having an event this Saturday for a re-release of their first anniversary brew its called the "512-One" it's a Belgian strong ale. You get 3 free tastes. Anyway a link to the event is below. I believe they are now delivering their beers to San Antonio, so look for them.
There is also a brewery called Independence and their beers are ok. Their best beer is the "Stash" IPA and its not bad. You will undoubtedly see Shiner Beers from Shiner Texas, the most famous being Shiner Bock, its ok. There is also a brewery called "Real Ale" they are not far from Austin in Blanco Texas, they make a Golden Ale called "Fireman's #4" which is pretty decent, I drink that at bars that mainly have BMC on tap.
If you are driving down and you need any kegging parts, Keg Connection (a highly praised vendor on this site) is in San Marcos (Between San Antonio and Austin) and they let you pick up from their warehouse, so it saves on shipping fees.
Anyhow, I hope you have a great trip!