Why don't you try Austin on Saturday the 19th? I used to live in Austin and made the drive to SA a few times. As said above, I wouldn't try it during the week unless you love sitting in traffic. Just try to take off early Saturday morning around 8 or so and head back in the evening.

I would head over to the UT campus if you want to go to a museum. The Blanton is on the south end of campus and the Bullock State History Museum might be interesting. There are a couple of cool restaurants on Lady Bird Lake that would be cool. Mozart's is a cool coffee/specialty desert place off of Lake Austin Blvd, it's west of downtown. Their deserts are awesome and it's right on the water which might be fun. I'd check out some stores/restaurants on South Congress as well, Hopdoddy, Homeslice, etc.

There's really not many "western" themed things around downtown SA and absolutely none in Austin. Tex-mex, well that's SA's bread and butter so that may be a little difficult to avoid. No offense to the SA members here, but I would do SA in a day and Austin for the week.