Hi San! Is this your first trip to San Antonio? I've lived there twice (Air Force wife here) for 3 years each time It's a wonderful city with so much to do and the River Walk is beautiful.
August is very hot in S.A. with temps usually in the mid to high 90's and often 100 plus degrees. I guess you are getting a taste of that weather right now. I would pack skirts and lightweight summer dresses with sandals. The evenings are more tolerable when the sun goes down. There's lots of great nightlife along the River Walk so be sure to include a nice outfit or two for the evening. You should probably also pack a summer-weight cardi for when you are inside with AC. I'm sure the Austin gals can give you some more guidance.
There are tons of great restaurants along the River Walk, including tex-mex, and the food is fabulous. Great shopping, too! Check out the Rivercenter Mall right there on the river, Market Square, and La Villita, a historic arts village on the south bank. Be sure to have a margarita for me and don't miss visiting the Alamo, it's within walking distance.