Check this one out! It looks easy but it will kick your butt, well your core!
Start with your hands and feet in a push-up position, bring one knee towards your chest while rotating underneath your body. Return to the starting position by reversing the movement. Repeat on the other side. If you find this to easy, add a push-up in between each set. Don’t rush this one, your goal is to get your thigh parallel to the ground.