Typically, people believe that the guaranteed quick route to obtaining a rock-solid six-pack is a path packed with hundreds of sit-ups or abdominal crunches. Wrong!
The reality is that even if you do crunches every day you aren’t guaranteed to get the stomach you desire. Getting those highly sought after, toned abs requires more work than just abdominal exercises.
Here are some alternatives to crunches!

V-Up
This one is pretty fun.

Start position: Lie back onto floor or bench with knees bent and hands extended towards ceiling. Head should be in a neutral position with a space between chin and chest.
Leading with the chin and chest towards the ceiling, contract the abdominal and raise shoulders off floor or bench. Also raise legs up towards ceiling and attempt to touch your hands to your feet.
Return to start position.

Alternating Toe Touch
I love the way this one looks

Start position: Lie back onto floor or bench with feet up into the air. Head should be in a neutral position with a space between chin and chest.
Leading with the chin and chest towards the ceiling, contract the abdominal and raise shoulders off floor or bench. Extend one arm and reach for the opposite foot. Return to start position and repeat with other hand.
Remember to keep head and back in a neutral position. Hyperextension or flexion may cause injury. To increase resistance, hold medicine ball in hands. To decrease resistance, position hand closer towards body

Plank
Always a classic

Start by placing your forearms on the ground and forming a plank with your forearms and feet.
Hold this position keeping your body parallel to the ground for the required time.

Burpees
Jump up, up and up

From a standing position, jump as high as possible and land down on your feet with your hands on the ground.
Kick your feet back. For a moment you will be in a push up position and jump back up again as fast as possible.
Repeat for the required repetitions.

Belly Blaster
So for those of you who are normally on top.....

Start in a plank position with your elbows and forearms on the ground, your body parallel to the floor and up on your toes.
Make sure your belly button is drawn in toward your spine and raise your hips up into the air and hold for a 1 second count.
Return to the starting and repeat for the prescribed number of repetitions.

Supine Double Leg Raise
So you'll be able to put your legs on his shoulders with ease

Start by lying on your back and a slight bend in your knees.
Raise your legs to a perpendicular position to the floor. This is your starting position.
Slowly lower your legs until you are no longer able to keep your abs tight and your low back on the floor.
Raise your legs back up to the starting position and repeat.
Only lower your legs as far as your abs have enough strength for.