You did say San Antonio, and Fredericksburg is 70 miles away. But if you have any interest in World War II history, I agree with above that a visit to the
National Museum of the Pacific War is worthwhile. And Fredericksburg itself is an interesting town with all its German history and architecture. And shopping galore. And extensive winery tours if you like that.
A person could spend days just touring the hill country towns of Fredericksburg, Kerrville, Boerne (pronounced bernie), Comfort, Bandera (“cowboy capital of the world”), and many others each with their own personality.