I love the Cardio Abs video from Insanity.  I always felt like that was the best ab workout I’ve ever had.  So instead of doing Legs during this “split”, I decided to do an Abs Day.  I fast forward through the “cardio” portion of the video, and go straight to the C-Position Abs routine.  It’s a lot of holding position, and turning to work your oblique muscles.  Then there’s a set of plank exercises too.  It’s awesome, only takes about 10 minutes, and I feel really good afterward.