The chemistry of cooking is a good topic to explore. Recently my daughter asked the question, "how does the quantity of salt impact the rising of bread?" Her teacher originally rejected the question as a science fair topic, saying that she had checked with a proficient baker who said that salt would not affect rising. We discovered through tightly controlled experiments (including measuring ingredients to the tenth of a gram!) that she was dramatically wrong!
What is interesting to me about this question is that it relates directly to everyday experience, and is in an area that at least my child can relate to: bread making. The chemistry of cooking has innumerable possibilities that can benefit from the rigor of scientific analysis.