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.