I'm not sure what would be particularly interesting to you, given that you have already traveled to places that are far more interesting. However, here are a few things:
The only thing in Austin I know about is the bat tour. There is a bridge that is home to something like 1.5 million bats. Various boats will take you out just before dusk to watch them, and every night like clockwork, all of those little bastards come out in a continuous stream for about 20 minutes, until they're all out. You'll need binoculars to see more than an amorphous cloud, but I thought it was pretty cool. I forget the name of the bridge, but velvetdoughnuts can tell you.
San Antonio: The River Walk, iirc, has always been a commercialized tourist trap. But it's a very pretty one. There's a boat tour that will take you through the entire area and tell you about the history. It's worth taking at least once. As vm will recall, I am unimpressed by the faux-Mexican restaurants there. He liked the food at the place I took him to, but the poor bastard just doesn't know any better, bless his heart. They make good margaritas, but that's about it. The taquerias a mile or two outside the River Walk have much better food, and they're cheaper, too.
I dunno how you feel about gawking at animals other than alligators, but there are various animal attractions in New Braunfels and the outskirts of San Antonio. The San Antonio Zoo is decent; not the best, but decent. They have a nice big alligator pit. In New Braunfels there's a Wildlife Safari place that has an old-fashioned drive-through animal preserve. You can see lots of African and North American critters there, including ostriches. There's also the Snake Farm in New Braunfels, which is a small mini-zoo (but well maintained with professional herpetologists on site) that includes one big room full of snakes from around the world.
Finally, there's a big ranch in New Braunfels set up like a frontier town. Not my cuppa, but just in case you want to see a Texas Longhorn up close ...
I'm going to be in friggin' Oregon (Depoe Bay) for the first part of your visit, so I dunno if I can meet up, but I'll see if I can swing it. Where you're going is a good 3.5-hour drive from where I live, so I can't make promises.