Definitely spend some time in Austin if they’re near there, fab city. The Capitol building, LBJ presidential library, bat bridge. Some good food places too.
A ranch might be fun, there are lots around Bandera.
Hill country- Fredericksburg, Luckenbach, Greene, Wimberley, Tubing on the Guadalupe river.
San Antonio is a very small town but if you’re nearby a quick stroll along the river walk or trip to the Alamo is interesting (but it’s very small!)
Coastal- south padre island - but that’s very far south so not as accessible.
Galveston has its own charm, but don’t expect miles of beautiful beach. Easy to combine this with Houston and NASA. Houston as a city not the best holiday destination but quite good for sports (baseball, basketball, football), and it’s nice around Hermann park for the zoo and museum district. Then NASA and access to Galveston.
I like Dallas - again, small quite compact city. The book depository is interesting, and other good museums. Nearby Fort Worth is fun for a wander too.
Waco is an interesting small town as a detour between Houston and Dallas, Magnolia market was always quite popular.
You’d need a lot of time to do all that properly though!