C.S.
How about a project where you test which brand of batteries really does last the longest. This is easy, but fun and interesting. You just simply buy 4 different brands of batteries, energizer, duracell, etc. And then buy 4 of the same cheap flashlights (dollar store probably has them). Load up the batteries, be sure to label which flashlight has which battery and then leave them all on until they start burning out. You'll know which one lasts the longest. Just a thought.
You could also do one on growing grass seed. Which environment does grass seed grow best? Fill up 4 styrofoam cups with potting soil, sprinkle in grass seed, add water. Put the cups in different environments throughout your house. (refrigerator, sunny kitchen window, dark closet, the attic) See which one grows fastest.