Fill your gym bag or carry-on suitcase with books or something else heavy enough to create a challenging weight. Hold the bag by its straps with both hands. Let your arms hang straight down, with your hands 12 to 24 inches apart. (the farther you place them apart, the easier and more natural the exercises is for your shoulders.)
Lift the bag straight up along your torso until your upper arms are parallel to the floor. Pause to feel the contraction in your delts and traps, then slowly return to the starting position, staying in control.