Well, I got her good.
As I suspected, since I had stuck two of the boxes together and tied them up pretty so it would be kind of two for one (we go around in a circle one at a time so everyone see's every one elses gifts) she'd got greedy and go for it early on. She unwrapped eagerly, and then opened the glittery gold box. Her face was priceless when she realized there wasn't anything in the box.
"Last year you said you wanted boxes."
I got a lot of looks and snark after that. She didn't get to her real present until later (which she liked) and I told her that she couldn't possibly think I was really that mean and would only get her empty boxes.
The funny part was my other cousins son, who's about 5, was really fascinated by the boxes and really liked the padding that came inside them. He kept opening them up to fuss with the padding.