The reverse plank is another great exercise that works the core. It targets the muscles at the back of the core and the obliques.
Sit on your mat with your legs stretched out in front of you. Straighten your arms and have your fingers pointing forward towards your toes. Push up using your arms with your hips lifting towards the sky. If you have weak wrists come down on your elbows. Hold your core tucked in and remember to breath. Try to hold for 30 seconds to 1 minute. To make it more challenging you can lift one leg up.