Chest fly is another way to work your pectoral muscle, this is one of my favorite chest exercises. Chest fly make a nice complement of push up and chest press. In order to avoid hurting your shoulder is you'll need lighter weight to do this exercise. Also take care when lowering your arms.
Since my I like to challenge my core, I prefer to use exercise ball instead of bench. But if you think your core is not strong enough, you may use the bench or step.
A. The starting position is lie on the ball with your upper back and head supported by the ball. Grasp dumbbells in both hands by facing to each other.
B. Keep slight bend in elbows and stretch dumbbells out to side.
C. Slowly return to starting position ( A ).