It is my understanding that with a CS breakfast, you will get 1 entree and 1 non-alcoholic beverage. A bakery item would count as a snack credit so I assume that no, you would not be able to substitute your beverage for a bakery item. It sounds like many people stock up on bottled beverages when available (water, milk, or juice) to use later on in the day or the following morning to add to a "quick" breakfast. Your mug is only refillable at your resort, so if you are not eating breakfast at your resort, you will probably want the beverage that comes with breakfast anyway.