I have a 4th grader and a kindergartner doing science fairs this year. My daughter (the 4th grader) has done a science fair every year, this is my sons first one.
When my daughter was in kindergarten I think we tested batteries to find if there was a difference between expensive batteries and cheap batteries. Used some AA's little plastic battery holder from radio shack, and some 1.5 volt motors. Took voltage readings with a hand held voltmeter every 15 minutes or so until the motor stopped spinning. result was something like the duracells lasted 3 hours longer than radio shack batteries. the radio shacks looked good at first but once they started dropping, they dropped off fast. She took 2nd place in kindergarten.
This year my son is doing crystals. just epsom salt and water, letting the water evaporate and leave behind crystals. Were going to try to do it with a few cups and let him determine what the difference between large crystals and small crystals are by seening the difference between one "grown" in the counter, one outside and on int fridge. The idea being that crystals form different physical characteristcs based on the conditions as they form. From that, you can look at a piece of quartz or granite or the like and make some simple determinations about how the rock was formed, ie did it cool rapidly, slowly.
good luck, post back with your ideas. I love science fair time.