A plank is an isometric exercise that strengthens and tones the abdominal muscles and lower back. It requires no equipment and little space, making it one of the easiest ab exercises to squeeze into your day.
This is one of many variations you should try.
From a standard plank position, reach your right hand to touch your left elbow. Put it back down and repeat with your left hand tapping your right elbow. Do not let your hips sway side to side as you move.