San Antonio is a really cool city. My uncle used to live there. Be sure to see the Alamo and the Riverwalk! Especially when it's lit up at night. Sample some real Mexican food!
A couple of my favorites in Texas were the Battle of San Jacinto site and NASA Space Center in Houston. My visits were many years ago, not sure what they look like now. But it all depends on what kind of things you like, I had an interest in that specific battle. Not everyone will.
How set are you on Texas? From where you are there is a lot of south and this country is full of history and things to do. You could pretty well pick a destination then do a search for things to do in that area. I bet you could even find something of interest in Bay's area if you looked hard enough.
I could give you some of my favorites, like Laura Plantation near Vacherie Louisiana. That's the only plantation I ever visited that really told you how people lived, how boring life really was on a plantation for kids particularly. If you get a good tour guide they'll tell you stories about how horrible some people were and how good others were. They might explain how you could tell what language people spoke inside by the color of the building. That's a pure history tour, not just a "They had some pretty furniture" tour like most plantations. Can you tell that was one of my all-time favorites. We took the scouts there a few times as well as visitors from Europe and Minnesota.
If you decide on a destination let us know. Someone on her might know of some real gems in that area that are both interesting and educational for your Grands.