Breakfast--pool deck restaurant, sit down restaurant (names vary by ship). Goofy's Galley or equivalent will have yogurt parfaits and healthier options. Cove Cafe has pastries
Lunch--pool deck restaurant and sit down restaurant. On longer cruises, often a restaurant buffet as well (themed). Cove Cafe desserts. Fast food outlets hamburgers, etc. will be open all afternoon. Goofy's galley or similar will have healthier choices all afternoon (fruits, paninis, roll ups, etc.) Ice cream place will have soft serve.
early evening--snacks at Promenade Lounge/Diversions (or equivalents on other ships) + fast food outlets, Cove Cafe, Vista Cafe, ice cream
Dinner--pool deck restauant, fast food, scheduled dining
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late night--snacks in nightclub area + ice cream
Hours vary--check navigator. Room service is basically 24/7 except last night and during safety drill. If you can't find free food on a DCL ship, you aren't trying. When in doubt, call Guest Services and ask.