I love science fairs too. Right now I'm designing my yearlong project for my "biotechnology senior tech. lab" that involves the effect of differing ratios of auxins and cytokinins on drosera capensis tissue culture growth. Do you have access to column chromatography, sds-page, or gel electrophoresis? Those offer lots of ideas as do organisms such as fruit flies (drosophila melanogaster), flatworms (planaria), and daphnia.
Just a side note, my teacher told us today that several biotech. firms will be donating a refurbished DNA sequencer to our school (TJHSST)!