Herfnerd
Our last Alaskan cruise was this past September - a RT out of Seattle with stops in Juneau, Sitka, Ketchikan, Victoria BC and Glacier Bay
Make sure you have a good camera, waterproof outer later and good walking shoes - might not be a bad idea to waterproof them. For clothing, definitely layers - I wouldn't necessarily go out and buy anything special, either.
Artemisc
At Juneau, we rented a car and drove to Mendenhall glacier, and a rainforest, which was cool because I had no idea there were rainforests in Alaska. At Icy Strait, we booked a whale watching tour with Hoonah Travel Adventures. The group was maybe 16-20 people. As opposed to the ship tour, which was a hundred or so people on a huge bar. We saw a lot more than the ship tour people. At Skagway, we rented a minivan and drove into the Yukon (Canada) and went to Emerald Lake. It was an amazing drive.
We cruise a lot, and try to snorkel at least once on every cruise. In keeping with that tradition, we went snorkeling in Ketchikan. Can't say I'd do it again, but I can say I snorkeled in Alaska. As for clothes, you can't go wrong with stuff from Columbia. I went to an outlet and got a breathable rain jacket, fleece, waterproof hiking shoes and a waterproof backpack, all for about $100. It's all about layering. The temps ranged from 42 to 78.
Gypsyfish
Here's a blog where someone gives suggestions from their experience- https://tatertotsandjello.com/12-must-have-items-to-pack-on-an-alaskan-cruise/
I'm taking some water-proof gear- jacket, pants and shoes, as it rains a lot in the summer.