On a cruise? I would get something that they can do if they don't feel like going out on the deck, pool, etc.
A package of cards - even Uno -
Travel Yatzee - like this - http://www.amazon.com/Travel-Yahtzee/dp/B0027SG2VO - I have one but didn't pay $20 for it...I'm sure you can get it cheaper
http://www.target.com/p/yahtzee-steal-the-deal-dice-game/...
http://www.target.com/p/world-series-of-yahtzee/-/A-14298...
Any ways - my 11 (almost 12 year old) would love to have card games, Magic Cards (hard to know if the kid receiving likes this) but packages of Magic Cards would be awesome for my son...you can pick those up at Target as well.
Not sure if a package of baseball cards would work...kinda like the Magic cards...not sure if the kid is into baseball...or basketball...or football...
Does this sign up give you ANY information about the kid(s) and what they like and don't like? I guess that would help.
Cinch bags - go to the 5&under store on Route 7 (Burlington Coat Factory) and look at what they have in there. Nothing over $5...
Good luck!