First (this one's explosive) fill balloons with Hydrogen, oxygen and Helium gas. Light on fire with a candlestick (they do this at Cal Poly SLO all the time, so be careful, but it'll be ok) some explode, others wont, Why?
Test the water quality in your area. People love this because it reveals all sorts of nasty stuff in the tap water at parks and schools. Get a test kit and do basic chemical tests.
Put a egg in vinegar. the shell will get eaten away and it looks like jello. compare it to other acidic things like coke, and other sodas.
Put chicken bones in various acidic foods... see which one disolves fastest.
See if plants respond to playing music. play music to plants (the vibrations actually make them grow faster), see if the types of music make a difference.
You could do social science experiments such as having your male friends walk around with purses (how many get arrested at the mall... funny my boyfriend tried this once and actually did get talked to about possibly stealing)
you could combine Lithium metal and water. also explosive.
Discover the power of hydrogen peroxide (its also explosive in high quantities). heat of reaction has to do with the amount of energy stored in chemical bonds... what is its heat?
ingestion of styrofoam causes cancers. Does hot water (ie coffee and tea) really dissolve styerofoam cups (and yes it does).
find the Ld 50 of various products (its the amount that is needed to kill of 1/2 the population of rats) such as coffee and blue dye in gatorade
seperate the colors out of gatorade dye
make beer (it's fermentation in action!... but buy a kit and have your parents help)