Don't forget you will have gratuities added to your account. Last I heard it is $10.50 per person per day. Thus for two people on a 7 night cruise, you will be paying $147. The remainder is $103. Most people are likely to buy something or a few cocktails, etc.
You could always buy something in the gift stores to give as gifts when you return home. I doubt many people would have much left over from a $250 onboard credit. But that is just me.