Mapping the different phases of the moon will make an interesting science fair project. In fact you can make a working model with a bulb powered sun and a highly reflective surfaced moon in the science project model. Think of the basic solar system model and draw inspiration for the design. You can always rework some elements as you go.

You can restrict the model to merely the sun, the moon and the earth to simplify it.  You can follow the phases of the moon from Full Moon to 3 Quarters to Crescent to New Moon with the help of diagrams. Make the rotation of your celestial bodies possible with a powered motor to make it more impressive.

Or just use your hands to move the spheres around to get the lighting on the moon to reflect all the phases from the sun. Use the lower portion of the model for the Earth. Have the sun fixed in a specific position and allow the moon to rotate around the sky region of your model.

And if all that sounds way too much effort to do, just go and blow up a volcano! After all you are supposed to have fun too. Maybe after you have done a simple project and done it well, you may feel challenged to do something even better. That’s when you can begin work on more complicated science fair projects.