If you plan on taking camcorders, digital cameras and such, take an extension cord with a three plug Y. (Cabins usually only have one outlet outside the bathroom.) One for the camcorder, one for the digital camera batteries and one for the laptop to download the digital cameras to.
Also, while the ships have irons in the laundry rooms, use them early in the cruise or bring your own. There is a substantial line each formal afternoon. By the 3rd day of the cruise the wait for a washer or dryer is way too long. The Ocean Princess class ships have on-board laundry at very reasonable prices - less than most hotels in major cities.
Most excursions will provide rain gear, but carry rain coats or shells for the deck for College Fjord and Glacier Bay cruising and for getting to and from the excursion and for walking around shopping.
Also take an extra empty duffel type bag to bring back souvenirs. Extra shoes are a must. For outdoor type excursions take a backpack with a dry pair of socks, a pool towel and a dry T-shirt for that accidental slip into some cold water. Also great for carrying cameras, film, batteries and a water bottle.
Don't forget the bug spray and sunscreen.