If you are on the 7 day cruise (Magic), most evenings they serve a casual dinner in Topsiders, the buffet restaurant on deck 9. As I understand it, you go through the line to pick out appetizers, and tell them what entree you want, then they serve you the rest of the meal.
If you're on the Wonder (or just want other options on the Magic), Pinnochios and Plutos (the fast food type things) are open for pizza, hot dogs, burgers, chicken strips. etc. The navigators that are listed on Dave's site (dcltribute.com) will show the exact times that they are open on a normal basis.
And there is always room service. They offer sandwiches, cheese and crackers, pizza, soup and a few other things 24 hours a day, free of charge (just please tip the delivery person).
Have a great trip!