I've written a number of hubs on this topic which you can check out, but to answer briefly, I think that science fair projects involving food like candy are very popular with elementary kids and lend to a nice board. One of my daughters did one on which kind of chocolate melts the fastest. Another one done at our school investigated what was the percentage of different colors of candy in Skittles, M&Ms etc. For older kids be sure the project follows the scientific method--I have a hub on how seed germination is affected by salt in water. Or try leaving out one ingredient in baking cookies or muffins (this is like "knock-out" mice which have one part of gene disabled).
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