I have to write the same story I wrote for a similar question a few months ago.

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.

The non-profit Science Buddies websites can help your child find a project.  The have a project idea section, and some projects have very detailed instructions.  Or you can use their Topic Selection Wizard, answer a series of questions about the project requirements, your child's interests and due date, and the site will recommend some projects.  They also have great info on the scientific method and putting a project together:

http://www.sciencebuddies.org/science-fair-projects/project_ideas.shtml