The water magnets are always a hit. I give each kid 3 water molecules and ask them what it is. We can talk about polarity and Hydrogen bonds between the molecules versus the covalent bonds that hold the Hydrogens to the Oxygen. The set also comes with salt - we talk about how one is positive and negative and that is what holds them together.
Then I have the kids get in groups of 4 with the people they are sitting near and ask the kids to dissolve the salt in their water. It often takes a while to get hydration spheres.
For the first time I can remember I had students testing how strong the magnets were .... how many pages can you have between the pages and still be able to control the top magnet with the bottom magnet. I love curiosity.
I do not make any money from this product - I bought them from 3D Molecular Designs and I have the 6 cup kit.