Make a Lava Lamp. This demostrates that oil and water can/can't mix.

In a plastic bottle put some water, colour it with some food dye then pour in a few centimetres of oil. The oil will float on the top. Once the water is still and the oil is sitting on top sprinkle some salt over the oil and the oil will fall down to the bottom of the bottle in balls. As the salt dissolves the oil balls will float back up to the top. It looks pretty cool.

I have a link for this if you like. I don't know if I can post it here so PM me if you need it.