We measured 1/2 cup of water in one cup and 1/2 cup of alcohol in a second cup. Then I had them guess as to what the final volume would be. They thought, logically that it would be 1 cup. I am sorry that I did not get pictures of this, but once it got started, I forgot to take pictures. When they are combined, the result is just under 1 cup. We talked about how the alcohol molecules were a different size than the water so they fit together more tightly that if it were just one type of molecules.
I illustrated this concept by using some marbles, some popcorn and some sand. We started off with large "molecules" which were represented by the marbles. When I put them in the jar they could easily see that there was a lot of empty space in between the marbles or molecules. Then we added popcorn kernels and they took up some of the spaces between the marbles. The popcorn kernels represented a different kind of molecule, which can fill in the spaces between the molecules without increasing the total volume that much. Then we added salt and this filled in much more of the gaps, again without increasing the volume that much. The water and alcohol molecules did not fit as closely together as our marbles and popcorn, but the model served to explain what they saw happening in the experiment.
Sadie at It's a Boy's Life does a similar experiment but she has her boys actually measure the amount of marbles, popcorn and sand (she uses sand instead of salt) that fit together. She also has her boys make predictions of the amounts that can be added.