Experiments having to do with how light bends, reflects and refracts provide visible and interesting results, and many are simple enough for elementary students to perform with only a little supervision. Help children explore how light bounces off different combinations of mirrors, how objects dipped in water seem to "bend" due to refraction, and how curved lenses or mirrors affect the images that pass through them or bounce off them. Children may also enjoy playing with prisms to see how light breaks up into different colors.
Experimenting with sound gives kids an excuse to have fun making different noises. Letting them vibrate rulers hanging over the edge of a desk, pluck rubber bands at different tensions, or experiment with tuning forks or simple stringed instruments can help them develop an intuition for the relationship between length of a vibration and the pitch it produces. Another idea is to cover a pot tightly with aluminum foil and sprinkle salt on it, then let the kids make different noises (with a whistle, pot lids, etc.) nearby to make sound vibrations visible.
Gravity is an important force in physics. Let kids experiment with gravity by having them drop different objects off a short ledge or off the edge of a table. Help them notice that large and small objects take the same amount of time to fall unless air resistance slows them down, as with feathers or unfolded paper napkins. You can also use toy army men with parachutes to demonstrate how air resistance and gravity oppose each other.
To learn how forces interact when objects collide, you can let kids play with billiard balls or toy trains that connect with magnetic hitches and run on straight tracks. Help them notice that when a moving train car collides with and attaches itself to a stationary car, the total speed of the two cars is lower than the original car's speed. Another experiment is to put two dolls in a cart and tape one in as if with a seat belt, then let the cart crash into a wall or fixed object so that the unsecured doll flies out of its seat.