I sprung for the light up cups too. And a couple of other Smoothies for my teen-aged sons. (Sometimes they are free in Vibe and Edge, I think). You certainly don't have to spend extra for food and drink on the trip if you don't want to. There are so many free choices on board.
We loved the selection of free food from room service. If you search the threads, you can find where people posted pictures of the room service menu. Most of it is free, except as mentioned above, pre-packaged snacks and cans of soda.