Simple distillation - distill red wine, you then end up with 2 products which you can easily show are different 'cos one is flammable (and the other one puts flames out) When teaching about mixtures, and how mixtures can have different components in different ratios, I used water to demonstrate the point. I explained how tap water and mineral waters have different quantities of minerals inside them, showing a label from a couple of different brands of mineral water. I explained that this might affect the taste.

Make breakfast in front of the class! Fry an egg, burn toast, have some 'gone off' milk for them to smell, ice for your orange juice, butter which you 'accidently' allow to melt by being too near heat etc etc. You can also make tea and add sugar. Good demo, but your lab will smell of burnt toast all day!