Popping balloons is a lot of fun. You can use this to show physical laws in action by putting a couple pieces of tape close together on a filled balloon, then putting a small hole in between them. Perhaps surprisingly, the balloon will not pop. The reason for this is because a balloon popping is actually an extremely fast-moving tear. When you put tape on the balloon, it stops the tear from moving. This shows how catastrophic crack propagation works.
Collect a few different objects on a piece of wood and try to push them. Estimate how much force you are using. If you can push something with one finger, you don't need much force; if you need to put your back in it, you are using a lot of force. Discuss why different objects -- particularly objects with similar weights -- are easier to move than others. The reason is because of friction, which is the amount of resistance created by the different materials involved. Ask children if they can think of ways to reduce friction, such as rubbing baby oil on an object.
Have a child sit on a swivel chair while holding a can in each hand. Have him put his arms out to his sides, then spin him around. While he is spinning, have him bring his hands in to his chest. He will start to go a lot faster, much like a figure skater does when she is spinning on her axis. This shows how angular momentum works; an object has less angular momentum (and therefore moves slower) if it is wide. So, by making himself narrower but not changing his weight, the child reduced his angular momentum and therefore began spinning faster.
The relationship between mass and acceleration is a key component of physics. Basically, objects with smaller mass have less acceleration when given the same amount of force than objects with larger mass. Put a few different objects that are easy to move on the ground. Make sure that the masses are quite different, as this is an easier variable to control. Push each one with about the same force and measure how far each one travels. A marble will travel much farther than a brick, as it has lower mass and therefore accelerates more under the same force as the brick.