This is inspired by thinglab, an early Smalltalk system back in the mid 1980's. You have a window divided into a small palette and a larger playing area. Objects have properties. Once in the playing area, you can click on an object to change its properties. A ball has mass, elasticity, direction, size, etc.
Objects interact with all other objects in the playing area.
For example, you can create a ball and a set of walls, and put the ball in motion so it bounces around the walls, absorbing energy until it stops. Of you can create two balls with a lot of mass and set them moving, and they will capture each other in their gravitational attraction and start circling each other. Or you can lay a bunch of pegs down, shoot a ball towards them, and make a pin-ball game. Or you can lay down a peg, put a string on the peg, put a ball on the other end, start the ball moving at right angles to the string, and watch it circle around the peg. Or create a ball with no mass that holds a spring that holds a ball that holds a pen that is suspended over a wall.