The picture below shows a missing object when I brought the plate back. I told my son to figure out what was missing and I told him how many items were missing and although it took him a few minutes, he eventually figured it out. With a big grin he said "Do it again Momma!" And so the game lasted about 10 minutes, which is more than I expected.
I also did this game with my 10 year old cousin who was hangin' out with us today. I put a lot more objects on her plate and she had just as much fun with her game as my son did.
Enjoy! And please post any variations that you come up with; we'd all love to read them!