What makes a science fair project fun is your interest in it. Think of things that interest you, and then see if you can't come up with a theory to test relating to it. Like I think things with plants or bugs are fun, because those are the sorts of things that interest me.
One year we did a study of how earthworms in the soil affect plant growth and prosperity. We did 12 pots total, to buffer against any abnormal death or destruction, but you could probably get away with less. 3 control 1 gallon pots filled with top soil and a light mulch, had no earthworms, 3 pots had 3 earthworms, 3 had 6 earthworms, and 3 had 9 earthworms. If time allows, it would be best to start with seed, but we started with very young nursery 6 packs of snapdragons (any flowering plant will do), making sure, and documenting that all got the exact same exact care and exposure. Then every two to three days we charted the differences in leaf and flower development, and growth. Note any differences.. Color changes, pest problems... Etc. Take pictures at different stages of development and to note any changes. For the exhibit, we repotted one of the plants (the nicest one.. With added earthworms for effect) in a glass "pot" so that the soil and earthworms would be visible during display. Wrap the glass with black paper when not on display or the earthworms will avoid coming to the sides and leaving tunnels like you want then to. It was fun and pretty easy after we got it all set up.
That's just an example, find something that you're curious about and test your hypothesis. Remember that your grade isn't based on whether your experiment fails or succeeds, it's based on how thorough and careful you are in making sure that you follow the guidelines set out for the project. Good luck... And have fun!
