I have a soft spot for gravity simulators, my first BASIC program that wasn't a variation on INPUT "Your Age";A :PRINT "You are ", A*52, " weeks old." was a gravity simulator with a rocket in orbit and your capsule needed to dock with it by apply forward or reverse thrust (2D only!) What made mine a bit different was that you launched from the planet so you had to take into account where the other rocket was before launching or you wouldn't have enough fuel to dock.

What this example in Elm and all reasonably implemented simulators provoke is a sense of awe in just how hard it is to "drive" in orbit.