Sweatshirts, jackets, t-shirts all work for Alaska. If however, you are doing some glacier excursion, then possibly a warmer jacket will be required, but for going around town, etc...just dress in layers and peel off as needed, days start out cold and do tend to warm up. Don't forget to pack a pair of shorts, we went late in the season, first week in Sept., and had some afternoons in the 70's, so shorts were fine.
We had a variety of weather.. hot to cold. Zip off pants are super. LLbean sells them. Dress in layers. Shortsleeve cotton shirts for daytime, a light sweater or sweatshirt, a polartec shell, and a waterproof jacket with a hood. Cotton gloves and apolartec scarf. Trust me. I've used them all. Pack in ziplock bags.. Squeeze out the air by sitting on the bag just prior to the last closing. The times that I'v forgotten any of the above, I've bought them on the ship. They aren't very expensive and do make nice souvenirs. Enjoy. Also, take insect repellant. The state bird in Alaska is the mosquito. That's not a joke. They are awful.