Are your kids lookers or doers? Mine were doers. Looking at things just didn't satisfy them. So, yours are lookers, then go to Downtown Disney, ride the monorail, visit other resorts or take a boat ride. If they are doers, then go swimming, play mini-golf, or visit an arcade or Disney Quest. (but the money spent in an arcade or Disney Quest could pay for another day ticket).
No matter what, you will be at Disney and you will have a wonderful time!