On I-10 from pretty much San Antonio to El Paso, it's 80 MPH. There are alot of open spaces between towns. With only one day, I'd suggest
Balmorhea SPwhere you can swim in spring fed pool to cool off. Was there this past July and it was nice. Then in the evening, go to Mc Donald Observatory for a star watching party.
McDonald ObservatoryIndian Lodge Try to book a room here if you can
Fort Davis then do this.
Everything I listed is within a few miles of each other and will give you more than a full day's worth of things to do.
You could always do I-20 to Pecos Texas, drop down to do the things I mentioned, then pick back up on I-10