What about getting some special family portraits on this special date.
That's what I was going to suggest. You know those special boutique type ones where they charge you an arm and a leg. You could even say you won it as a prize in a raffle and have a limited time to use it, so you can't change the date.

Is there a particular reason why you want your wedding to be a surprise? She wouldn't be the type to get upset by that or is she kinda of expecting it?