I am starting my Science Fair and I need some ideas?

Well, I have an idea but I need some others. Thanx :)

How about demonstrating the effect of mocro-wave radiation on plants. Get a bag of seeds, have a control group that doesn't get put in the microwave and groups that are put in for 10, 20, 30, 40, 50 seconds. After the seeds have been exposed to the radiation, plant them and photograph the results when they have grown about six inches. You will clearly see the effects.

i know this is a bit late but depending on what grade your in my recommendation would be to build a railgun or a working mini mag-lev train.
obviously you wouldnt want to do the railgun in a middle school or even in some high schools...

the mag-lev is easy and cool, but the rail gun is much more impressive and scary... shooting through a couple inches of steel beats a volcano any day.