Shelly, welcome aboard! I love your question, because the answer is what makes Castaway Cay one of my favorite places in the world!
Castaway Cay is Disney's private island, available only for the use of Disney Cruise Line's guests. Onboard the Dream you will use your Key To The World card for any additional purchases you make. This card also works on the island for the shops, adult beverages, snorkel rental, and many other things.
You'll find two open-air barbeque dining locations:
Cookies and Cookies Too. All of this is included with the cost of your cruise! No need to spend additional money on dining on Castaway Cay or head back to the ship for lunch. Disney has taken care of it all. Just relax on the beach, grab lunch or a snack at Cookies, and don't worry about paying for your meal. Of course, for the 21 and older crowd, there are four bars to stop by. You will need to use your Key To The World card to charge any alcoholic drinks to your stateroom account. By the way, I highly recommend Castaway Cay's signature frozen drink, the Konk Kooler.
Have a fabulous time as you continue to plan your exciting cruise with Disney this spring, and let me know if I can answer any other questions for you, Shelly!