Give me some ideas for an elementary school physics project. I'd like my son to do something really unique, that the other students would not think of!
Sure thing! Here are some great ideas for an elementary school physics project:

1.) How do waves work?

Learn about sound waves and the concept of resonance by creating a 'glass harp' from glasses filled with different amounts of water. Why do glasses with some amounts of water produce different sounds than others? Will the glass always make a sound, or does it need a specific amount of water?

2.) What is the best type of paper airplane?

Learn about aerodynamics in this experiment. Make three types of paper airplanes (you may need backup copies to replace damaged ones). One of the planes should aim to maximize lift, another should aim to minimize drag, and a third should aim to be balanced between the two. Perform a drop test, where each plane is dropped from a set height and the record the times for each. Perform a throwing test, where the plane is thrown, and the distance and time it is in the air is recorded. Which went the furthest? The longest? Try performing performing a different trial where you throw much harder or softer. Do the planes have a preference?

3.) How, and why do car and bike brakes work? 

Learn about how bicycles and cars use brakes to stop by converting their kinetic energy into thermal energy using friction! Test this out by propping up the back wheel of your child's bike and have them pedal as fast as they can. Then, brake! Quickly examine the brake pads- they're hot to the touch! Experiment with and film how many rotations it takes the brakes to stop when you cover the rubber in different materials like paper, duct tape, or dirt. Which was best? Worst?
These seem sort of plain... I want him to knock their socks off!  Can you think of anything more exciting?
Sure, here are some more exciting ideas for elementary school physics projects:

1    Rocket Launcher: Build a rocket launcher using PVC pipes and launch water rockets, experimenting with different fins, nose cones, and payloads.

2    Roller Coaster: Build a roller coaster out of foam tubing and investigate the physics of motion, such as velocity, acceleration, and friction.

3    Rube Goldberg Machine: Create a Rube Goldberg machine that accomplishes a simple task using a series of chain reactions and investigate the principles of energy transfer and mechanical advantage.

4    Electric Circuit Maze: Design and build a maze that requires an electrical circuit to navigate, using wires, batteries, and light bulbs.

5    Laser Maze: Create a laser maze using mirrors and investigate the properties of light, such as reflection, refraction, and diffraction.

6    Sound Wave Visualizer: Create a visualizer that displays sound waves using a strobe light, a speaker, and a water stream.

7    Wind Tunnel: Build a wind tunnel to test the aerodynamics of different shapes and objects, such as airplanes, cars, and buildings.

8    Magnetic Levitation: Build a levitating train using magnets and investigate the principles of magnetic attraction and repulsion.

9    3D Printing: Use a 3D printer to design and build a working machine, such as a wind turbine or a robotic arm.

10    Augmented Reality Sandbox: Create an augmented reality sandbox that visualizes topography, water flow, and erosion using a Kinect sensor and a projector.