Here is a real simple project that the kids will love. An adult should cut off the top of a water bottle so that you have just the part where you screw the cap on with a little bit extra. An adult should also cut off the neck of a balloon. Now just duct tape the balloon onto the water bottle top. At first we tried it with marshmallows (safety first - haha!). That was fun but not as fun as using small tin foil balls. Go outdoors and shoot the tin foil balls - they go really far! Make sure you don't point it at anyone - it would not hurt too much but you wouldn't want "to shoot your eye out".
You can talk about projectiles and try shooting different objects. Observe how objects that weigh more or less travel in different paths.