If you want a great core workout, stick a small child in a big, plastic laundry basket. Sit on the floor with your legs crossed like you did during storytime in preschool. Set the basket directly in front of you. (Keep the child from crawling out). With arms at each end of the basket, pull to one side of you and then the other at a slow but fluid pace. Sit up as straight as you can while keeping your stomach muscles contracted. (Stop when you've pulled a muscle.)
I should have a six-pack sometime this weekend.