Well, why not talk to them and say you’ll make the spa happen rather than buying a voucher? A gift card is a crap surprise so why not communicate and say that for Christmas, you’ll make their spa trip happen. Then you go and book it or make arrangements. Like you would before gift cards existed. Sort it out for them so they don’t have to do it themselves.

If she takes 50 bucks and uses it in groceries it’s because she needs them. If you arrange a spa day outright then it’s not a question of money.