here is my suggestion:
Have the person hug a large Swiss ball and ask to hold it tight. Then press against the ball in a variety of directions, asking the person to keep the upright posture and not be pushed around.
You can have your client hold on to a rope with some kind of weight attached to the end of the rope. Have them swing the rope around above their head. As the rope passes a certain point in the rotation, have them contract their core to provide stability against the rope pulling their body in that direction. Obviously, you can have them contract their core at any point in the rotation.