DO NOT DO A VOLCANO! It's classic, but unoriginal. I did my science fair project on: Is a plant able to germinate if its seed were boiled, cooked, or dry-frozen before sown? I got an A+ on my project with extra credit. Try to create your own like doing a survey on how many people support or do not support gay marriage or something. Or you can test whether or not energy drinks really work... Idk. Just don't do a volcano. My teacher gave a C minimum to anybody who did a volcano!
Do a liquid testing experiment. When showing it to the class ask your teacher to sit up front blindfolded. Tell her/him that she/he has to guess what it is. Make her/him taste hot sauce, lemon juice and all that stuff. After act totally normal when they remove the blindfold and explain how our taste buds work and all that :P