Surprise her by having her favorite dessert set up at home when you get back from the show? She definitely won't be expecting it then, and it's more sweet than spendy.
Your religion and your "enlightened husband" status mean one thing, you have a lot of guilt! I'm sure you will make it up to her sometime.
Years of academic research suggests that holiday related guilt is best eased by sending a post-holiday gift to your father.