This activity is a fairly deep exploration of the elements of Newton's Universal Gravitation. In the "Gravity Force Lab" sim, the force between two masses is displayed. Both masses can be changed, as can the distance between them. Mathematical patterns are developed and cobbled together until the full universal gravitation proportionality is constructed.
My lab activity, "The Laws of Attraction" can be found here.
PhET's "Gravity Force Lab" sim can be found here.
Here's the sim as it can be embedded: