Many of you may have heard of this already, but it seems fun for kiddos and adults alike. Do it outdoors though! It involves mentos candies and a 1 litre of soda, and a test tube of tube of some sort. The soda will shoot up around 6 feet.
I've got to try this.
http://www.komando.com/tips_show.asp?showID=9667
http://www.komando.com/tips_show.asp?showID=9667