Use a metal ball, make two holes, one filled with water and the other with mercury and place some objects in between each hole so that when the metal goes through it, it does some work. The metal ball rises with the mercury and falls in the water due to density. The work it takes to transport the metal ball ball from the mercury to the water and back is constant, (as the distance between the two holes is constant) but the work done when falling/rising depends on the depth of each hole.
So basically, the metal ball starts at ground level, goes in the water and falls doing work via levers and then is transported to the mercury when it hits the bottom to where it rises and does more work. This can occur forever, but is it possible for the work done by the ball falling or rising to be greater than the work done by transporting the ball?