This is a great exercise for your shoulders. All you need is one powerblock or dumbbell. You can also use a bar weight if you have access to that.
With your feet about shoulder width apart, stand with your back straight, core tight and gripping the sides of the powerblock (or whatever weight you are using). Keep a slight bend in your arms. Lift the weight up and turn as if you were steering a wheel, then lower. Raise it again and turn the other direction. Continue to alternate back-and-forth until you have finished your reps.