If you do other science days, two more fun things are:
-baking soda rockets (film canisters, baking soda, vinegar) have the kids make paper rockets to glue/tape to the outside and let the fun ensue :)
-density tank. Honey, water and oil. Layer them in a jar. Have the kids make different shapes out of tinfoil to see where they'll sink to. It looks nice if you put food colour in the water so it stands out