Lying Leg Raises
Lie on the ground with back flat and legs fully extended. Raise your legs off the ground and then lower them close to the ground and back up. The distance you raise them depends on your physical capability. You can try raising all the way up but if you cannot
Hanging Leg Raises
Hanging leg raisees should be a staple exercise in your calisthenics program. A lot of calisthenics movements require strength gained in this movement. These are also great exercises for building muscle in your abdominals. Depending on you strength there are two different ways to perform this exericse. The first is
Assume the same position as a crunch. However instead of stopping once you feel the squeeze in your abs, with a sit up you keep going up until you are sitting straight up.
Lay on the ground face up with knees bent and feet on the ground. Place hands behind your head. Using your abdominal muscles pull your body up until you feel a squeeze in your abs. Then lower yourself down and repeat.