A Hamstring Walk Out is a great exercise which predominantly works the core, but also has additional core strengthening benefits.
Set yourself up lying on the floor with your knees bent up and feet brought in towards your bum. From here push your hips up, so that you create a straight line from your knees all the way down past your hip and to your chest. Dig your heels into the ground and lift your toes up. Take several little steps out until you have extended your feet as far away as you can without dropping your bum to the floor and then walk your heels back in again.
To increase the challenge you can hold the extended position for a few seconds, as this is the most challenging point in the movement.