Do some research into past science fairs:
https://www.google.com/intl/en/events/sciencefair/index.html
see what areas are presented and what ones aren't and go from there.
Also if you're good with java and programming then you might check out the Open Source Physics website (
www.compadre.org/osp) where you can get some libraries to do physics simulations.
From there you'd need to find something to simulate.
Science fairs are looking for original research so you must look for something few people have investigated and see if you can come up with something new. I know that may be hard at first but check around and see what you can find.
Another area is in robotics. Doing something novel and unique with drones would be a cool project. Maybe a drone programming language or drone library coupled with OSP.
Then there's LEGO mind storms, Arduino and Raspberry PI that could be used to come with cheap lab equipment for physics.
Look at some novel inventions for third world countries. The gravity light was something that caught my eye.
Sometimes its good to have a compelling backstory with your project. As an example, there was an American Invetor show where a fireman came out dressed in full gear. He presented a sad story about a Xmas tre catching fire and a family losing everything. This got him thinking of how to avoid the tragedy and he came up with the Xmas Angel, an ornament for the top of the tree connected to a pressurized water tank. At the first sign of fire, the heat would be detected and the water released to drench the tree.
With that in mind, how about technology to protect a classroom from intruders, automatically close the door when gunshots are heard... or technology for the teacher to protect the class other which is safe enough for grade school kids...
You may have other areas in your life where physics could be applied to solve some pressing problem. You just have to look around...
