Yesterday I did some science demos at a kids holiday club, using the Coke-Mentos fountain. Previous efforts led to the post Developing science demonstrations that actually teach science. It is fun and cool to do spectacular demonstrations that cause kids to go "Wow!" and think that science is "fun". But these also need to be a vehicle to teach something about critical thinking and the process of doing science.
Small initiatives can help. For example, I had one child record the height of each of the fountain, that was estimated by the group. This emphasized that measurement, error estimation, record keeping, and comparisons are key parts of doing science.
Aside: Yesterday I thought the Coke-Mentos fountain was higher than last time, particularly for diet Coke. I suspect the fact that is was a hot day helped, increasing the solubility of the carbon dioxide?
We also did Film canister rockets which the kids always enjoy.
I found it amusing that the kids ran off and told their friends they were doing "rocket science".