My kids always prefered clipping their Sea Passes to a lanyard they wore around their neck. If you have one, bring it, or you can buy one on the ship. Good walkie talkie radios were a plus for keeping up with my crew. Camera, sunglasses, big floppy tourist hats, back pack/carry bag for pool time and excursions, sun screen, extra swim suit so you always have a dry one...
Don't be afraid to get involved with the youth activities on board. My boys always enjoyed hanging out with others their age.