Pass the Ball: All players in a circle. Ask the players to pass a mimed ball to others (one ball at a time). The ball becomes heavier, until it weighs a ton, or extremely light, extremely big (and light or heavy) or extremely small (and light or heavy). Actors need to show the ball's characteristics in the way it gets passed.
Check out the baby improv actor performing Pass the Ball (well, she did a variation called Eat the Ball).
MORE IMPROV GAMES AT http://www.freedrama.net/improv.html