If you buy that Carnival thermal looking cup (mine's red) it fits a whole can plus ice nicely. It's only about $5 if you buy it when you buy your soda card. (I just bring my old one with me now!)
We actually figured out if you're going to drink more than 3 sodas per day then it pays to get the card. If you can live without soda for a few days, then it would probably be a waste of money. I'm not getting any this time, my 2 sons haven't drank much soda since this past football season (thanks COACHES!!!!) and my daughter asks for it but I still say no.
Sovereign of the Seas, Royal Caribbean 11-17-03
Fascination, CCL 2-14-05
Fantasy, CCL 2-05-06
Elation, CCL 9-24-06
Carnival Victory, CCL 1-28-07
Sovereign of the Seas, Royal Caribbean, 9-24-07