I am personally very interested in dance and have had a few ideas of how you could incorporate tap into a science fair project.
I feel that tap could be linked to science in a number of ways. You could look into how dancing uses different muscles in the body, the balance that is required, mechanics involved or the health benefits of dancing.
You could look into which muscles are used when you perform particular moves. To do this you could hold a certain tap position and mark the muscles that have tightened and take a photo. To make this more advanced, compare this with a photo of you when you are not in position, to see the differences in your body.
You could either do this experiment on yourself, or use a volunteer.
With some tap positions the dancer will need a lot of balance. So why not conduct an experiment that changes factors that may affect someone's balance while they are dancing. Things such as covering their eyes/ears or changing their body position to see which factors contribute most to someone keeping their balance.
If you want to create a project to do with health, an experiment could be set up to see how many calories are burnt while performing 15 minutes of tap compared to another form of dance or exercise. You could also conduct some kind of experiment to do with body strength, comparing someone who has been tapping all their life to someone that hasn't.
If these idea are too simple you could look into combining a few of them to make a bigger, better experiment.