I use 2 paper towels on a wood or laminate floor. Focusing your gaze forward helps to bring the knees in far enough. I prefer to do this as a forearm plank - more challenging, but I think more effective.
Ball exercises core workout. I love my yoga ball. I find myself sitting on it al my desk sometimes. It does help my posture and works the core at the same time. These are great simple exercises to try at home.