Our kids FIRST trip to WDW was a complete surprise to them even though it took us two days to drive down.
NONE of us will EVER forget the kids reaction when they saw the WDW gate for the first time!!
For your surprise.....
Which is EXCELLENT by the way!
You need two small itty bitty boxes. One box holds the key to the car. The other box holds either an old WDW resort room key, we keep all of ours,

OR tickets to WDW.
Obviously the kids get the boxes.
EVENTUALLY.
We, errr, Santa, did a scavenger hunt last year for the kids BIG XMAS gifts. Funny thing, just today our oldest asked if Santa could do the scavenger hunt again.
So as was already suggested a scavenger hunt would be a good idea. We left little notes for both kids to find which led to another note which led to a new note until the found the Big Gift.

Really built up the excitement. Next time more notes to keep them going.
Another idea is to do this LAST. Let them get through all of their gifts which will be ho hum. Save these big ones for last. Which is what we did with the scavenger hunt. If you have seen A Christmas Story when Ralphe gets his Red Ryder BB Gun you know the effect on the kids....
My parents did this to me one year. They got the biggest box they could find. They filled it with balled up newspapers and FIREWOOD.

The present really looked nice a big. It was heavy. It was huge. It was firewood.

With an envelope with money inside.
So you could wait until the XMAS gift opening orgy is finished.
Hand them a note that starts them on the scavenger hunt.....
Which leads to two Big Huge Heavy boxes. Filled with firewood, coal, rocks, or something that the girls will just hate.
But inside The Big Huge box is a little box containing a key to a car and a key to a room.....
OR you could have the scavenger hunt lead to the garage. Inside they will find a wrapped car for one DD and a big poster of Cinderella's castle for the other. But I like the small box idea myself.
Later,
Dan