Pull Up Bar Leg Raise:

Jump up and grab onto a pull-up bar, hang there without swinging back and forth, and inhale. Keep a slight bend in your elbows throughout the exercise.
Without swinging back and forth, raise your knees until they are slightly higher than perpendicular to your body. Squeeze your abs and exhale. Slowly lower your knees back to the beginning position (resisting with your abs). You can use arm slings to help you hang on the bar if you wish, as shown in the second picture. To make the move more advanced, you can raise your whole leg up.