Try the plank crunch.
The plank position begins with your forearms on the ground and your body straight as in the push-up position. Start with your right knee and bring it forward towards your right elbow, then return to the plank position. Repeat this process with your left knee. This is one ab exercises that will target your whole core; which other ab exercises can not do.
Add bicycle crunches to your workout.
Put your hands behind your head, lie on your back with your legs raised and bent at a 90° angle. Bring your right elbow towards your left knee then your left elbow towards your right knee. Like traditional crunches, these should be done slowly and deliberately to get the best result.
Get cross-crunching, too.
Lie on your back with legs and arms diagonally out so that your body forms an “X.” Keep your arms and legs straight, and bring your right hand towards your left foot, then your left hand towards your right foot, being sure to lift your head, neck, and shoulders off the ground. This exercise will target your lower abs, making sure that your abs get a well-balanced total workout.