Target had a small toy that makes creating this fountain really easy.
This is what happens when you mix soda and mentos. In his stocking Shaun received a small tube with a pin at the bottom attached to a string. You fill the tube with mentos and attach it to the top of the open soda bottle. The pin keeps the mentos from falling in immediately. The string allows you to pull the pin from a safe distance. Once the two of them mix, POW, soda geiser.
Hint: if you try this at home (and I suggest you do) use diet soda, it isn't sticky and can be easily rinsed off concrete or people.