For Quick Service Credits ONLY - You can swap the drink and/or the dessert for another item that is listed on the menu with the snack credit symbol next to it.
So yes, at BOG you could swap your drink for a cupcake.
At lunch & dinner you get both a drink and dessert with your meal, but at breakfast you just get a drink. For any of these meals the drink & dessert substitutions apply.
Basically, the way that I look at it is: Breakfast - choose 1 item with a snack credit logo. Lunch & Dinner - choose 2 items on the menu with snack credit logos.
It would be so much easier if Disney would list the quick service credits in this manner, but I assume that they prefer people to believe that they have to order a drink which costs Disney next to nothing instead of something more expensive.
Also note - that these apply to your quick service credits only and not any table service credits.