You can buy basic items at most (if not all) WDW resort gift shops. Items include half-gallons of milk, cereal donuts, chips, cookies, juice, soda, bread, etc. And you can get fresh fruit and veggies cut-up carrots and celery) from your resort's food court. The prices may be somewhat higher than at a grocery store, but not awful.
We always eat breakfast in the room (cereal, fruit, milk, juice), then eat most other meals in the parks. It is nice to have a few snacks in the room, though (chips, cookies).