A) check your cruise line's forum site first
B) Plan on Layering for daywear; slacks, Cotton Knit shirts L/S or S/L, sweat shirt or sweater, Polartec or fleece lined jacket, hat, gloves or headband for Glacier cruising and thick soled walking shoes. Leather sneakers are OK. this is the only cruise where your jeans would be ok. Evenings are the same as anyother cruise. If your ship has an indoor pool or hot tubs, don't forget your swim suit. Bring an umbrella.
Depending on when in Sept you are going- rain gear is a must, more rain as the month marches on. Knit hat and gloves, I disagree on jeans- I no longer bring them. A better option is lined wind pants and leggins. Jeans tend to get and stay damp, require a large amount of room and take forever to dry. I always do my laundry in Skagway and Seward to cut down on "volume". No winter coat or boots needed. I take my removable lined nylon jacket- lightweight and quick drying. and of course the trusty layers. Take 2 pair of walking shoes.
Very little hiking will be done maybe 4 of 5 miles tops. Intend to take the train/bus combo trip at skagway for sure, at present we have no locked in side excursions other than that. Any other good ideas from you would be appreciated.
What are your interests- there sure are a lot of choices in Alaska. Best would be to review a shore excursion list and narrow down your interests, any cruiseline list will do, I think Celebrity has one online?