The items that you will need cash for will be purchased off of the ship. All things purchased on the ship will be charged to your sail and sign card which will be added to your credit card upon debarkation.
If you have arranged tours or excursions yourself you will need to pay for these at whatever prearranged price set. As Dina said any shopping that you do while in port can also be taken care of with cash. Big ticket items such as jewelry ect. need to be paid for with a credit card in case any questions arise later.
$1000 is alot of green to be taking on a short cruise unless you have something to purchase in mind.
In any case I hope you have a great cruise and welcome to the "been bitten by the cruise bug club"