We always brought a 12 pack of Diet Coke in our luggage. We like Diet Coke (not Pepsi or European Coca Cola Lite). On our last Princess cruise to the Baltics (10 day), we were able to purchase a "soft drink" coupon for $25. It gave us unlimited fountain drinks (from the bar or dining room). NOT CANS!! It was perfect, they put a coke sticker on your id and you show it at the bar and you get unlimited fill-ups.
I will no longer carry the heavy coke cans on a cruise anymore.
We also bring our own bottle of vodka and have the room steward deliver a pitcher of orange juice and grapefruit juice each morning to our room. We then enjoy screwdrivers and greyhounds in our room throughout the cruise.