I did a bunch of research on what to pack, and how to get the best cruise experience... and some ideas were very useful while others we tossed overboard :) Here's my advice:
Some things to pack (in addition to clothes, toiletries, etc - check out the comprehensive packing list):
over-the-door organizer - with limited counter space, this was an easy way to keep everything organized and in reach
comfy slip on shoes - I got a pair of Sketchers GOWalk and they were perfect
team/school/town clothing - a great conversation starter
binoculars
games, if you want to play them with your group. We brought cribbage and Dixit (without the box)
sunscreen, after-sun aloe, bug spray, anti-itch med
Emergen-C and/or Electromix packets - really help if you start feeling under the weather, or dehydrated
lanyard - you can get the desk to punch a hole in your card and then wear it around your neck. Much more convenient, and harder to lose than if you keep it in your pocket. Majority of people do this.
rash guard - if you are going snorkeling or kayaking, you should get one of these.
decent map of the whole area you are cruising, plus more detailed ones of your ports. We didn't do this, and were wishing we did!
Take advantage of:
stretch class in the morning - it was a great way to start the day, though it's hard to do any yoga on one foot with the gentle rocking of the boat!
progressive trivia - fun and you get to feel more of a community since you see people consistently
art auctions - educational and fun
cruisecritic.com - you can get on a message board for your specific boat/trip and meet up with them and some of the crew one of the first days of the trip. They also have lots of tips.
wine packages - makes it easier (and cheaper) to choose wine at dinner
excursions sponsored by the boat - sign up early if there is anything you really want to do, or that might last until you need to board at the end of the day. The boat will wait if you are on an excursion sponsored by them but not if you made private arrangements. But if you just want to wander around town, or something short go ahead and do it on your own. Best to do some research on providers and preferably book it beforehand instead of relying on picking one of the many vendors hawking experiences on the dock.
laundry specials - on our ship, every 4 or 5 days, we could fill a (provided) large bag with laundry for $25, versus paying per piece, as at a hotel. Tip: roll up your clothes to make more fit!
the game room - it was pretty full each night. If you have a group, and like to play games, this is great. Our games were cribbage and Dixit!
Don't fret about:
coffee - you can make it in your cabin
hair dryer - also supplied
internet - take a break! If you need to check email (or if you want to do quick blog posts on the road!), you can use a kindle with 3G to do so while docked at most ports, for free though it is slow and klunky. They do also have internet on board for a price.
portable clothes line - there's one in the shower to hang swimsuits, etc.
"today only" on-board shop/jewelry sales - they will be going on for the whole cruise