You can get sodas that are served from a fountain machine during meal times, except at Topsiders Buffet or Beach Blanket Buffet.
At the 24 Hour Beverage Station on Deck 9 aft, they have coffee (regular and decaf), hot water and several flavors and types of tea bags (regular and decaf), milk, ice, and cold filtered water. They also have packets of instant hot chocolate.
If you buy the refillable drink mug, you can get it refilled for free at any lounge or restaurant that has a soda fountain machine.
If you'd like a soda that is served in a can, you'll have to pay for it even if you have the refillable mug, even at meal times. It costs $1.50 plus the 15% gratuity that is automatically added to your bill. You can pay for it using your KTTW card.
My DW likes Diet Sprite and it is only served in a can, so when she grew tired of Diet Coke from the fountain, we would occasionally buy a can for her. We had the refillable mugs as a benefit of booking our cruise through the
AAA agency so we didn't pay anything additional for the mugs and didn't feel that bad about buying an occasional can.