Real science fair advice from a sometimes judge:
1) You must measure something. (The Diet Coke went 39.2" in the air when Mentos was introduced.)
2) You must compare something to something else. (The Diet Coke went 0" in the air when a Hershey's Kiss was introduced.)
3) You must repeat your measurements enough to eliminate random chance as an element. (On 4 trials with Mentos, I observed 39.2", 34", 40.1", and 18" sprays of Diet Coke)
4) You should account for measurement variation. Gets more demanding in higher age brackets. (I surmise the 18" measurement occurred because there was not very much Diet Coke left in the bottle.)
Volcanos are dynamic art projects, not science fair projects - unless you figure out how to measure and compare stuff using your volcano.