Here's one for ya.
You'll need a swing.
And a Somebody. Or two.
And a timer or stopwatch.
An' that's it.
Object: To demonstrate that each swing of any pendulum of the same length takes the same amount of time. No matter the weight, and no matter object.
How to play:
Somebody hops on the swing, in the still position.
Start the timer for one minute.
Bring the swing back just a wee bit (a couple of feet, or so), and count the forward swings over one minute's time.
Put on your next (heavier or lighter) person, and pull them back the same couple of feet (for a slow swing) and count their swings over one timed minute.
Now to demonstrate a fast swing! Put on someone, start the timer, and count their high and fast swings over one minute's time.
Now switch people, again.
No matter who (or what!) is on that swing, it shall be the same.
Ours was always (Me, Maddie, Trev, and Air - fast or slow) 16 swings per minute.
How about that?!
This idea comes from the book Barron's Science Wizardry for Kids.