Make her wait.
If I were the Mrs in this situation, here's what would make me happiest:
1. If I had no idea a Big Surprise Gift exists. No hints, no teasing, no references to a big surprise gift -- nothing.
2. If I have a few gifts from you to open during Big Family Gift Unwrapping Orgy. Still no hints of Big Surprise Gift waiting.
3. If we have a nice conversation on the way home about how enjoyable the holiday was. Still no hints.
4. Arrive home. Feed the dog. And what's this? Big Surprise Gift!
Fun. Special. Time to use it. Time (and privacy) for sincere expression of appreciation.
But that's me. How does the future Mrs Dr Adequate handle/like surprises? Is she going to be comfortable with seemingly few and/or minor gifts from you in front of the family? Consideration of her feelings are much more important than my vote.
Whatever you decide, it sounds like a great gift that you are excited to give her. Anticipation -- hers and yours -- will make it wonderful.