I strongly recommend a card carrier that you can purchase on the ship for $5.00. I'm not into souvenirs, but I knew ahead of time that this would be my only one (other than photos).
It's watertight, it will hold your KTTW, a driver's license, some cash, and your kids' cards. There's a larger size that could hold approximately a deck of cards. My wife got this one. It was $8 at Mickey's Mates.
If you don't want to wear it around your neck, the string detaches easily and the case can fit in your pocket.
If you detach the string properly, it can also double as a really cool sling (ala David and Goliath) to be hurled at anything on the cruise that you don't like. I recommend using it during "Hercules, the Muse-ical".