We have spent a bit of time in Fredericksburg - not to far from San Marcos, you can make a day out of just visiting wineries... if that's what you really want to do. Top mentions - "Grape Creek" and "Heath" in the same parking lot (sch tastings if you plan to visit, Heath is all sparkling so start here first) , "Barons Creek" (one of my favorites), "Arrowhead" (they actually find arrowheads on the property and have live music on Saturdays sometimes also a favorite) "Rhinory' (they have real rhinos there, cool experience)"Slate Theory" is also really cool, has underground cellars and tasting rooms(sch a tasting if you plan to visit). "Safari" has a neat atmosphere, wine was ok. "The Edge" is awesome, atmosphere is great and shaded looking over the river - good afternoon stop. "Invention" is new, wine was excellent (I believe its owned by the same owners as Grape Creek but on the opposite side of Fredericksburg from the rest). "Altstadt Brewery" is also on 290, great beer selection and food is great, makes a good lunch spot with out going back to town to eat, recommend and early lunch 11-11:30, it can get busy.
Could be a fun run if the weather cooperates 