where are you going on your cruise? that would be helpful to tell you what to bring...
make sure to bring a credit card or something, they book an additional 20 dollars a day for gratuity fees.... plus also, make sure to bring some motion sickness pills... that was by far the worst part of the cruise, those open waters can get really choppy.
Also, I wouldn't book any excursions... the cruise liner charges you twice as much as if you just went and did it yourself... plus having one activity when you port as opposed to just getting to do whatever you want... on our next cruise, we won't book any excursions and just do what we want.