HI....went to Alaska two summers ago in August. Needed sweatshirts and jeans most of the time and definately the lightweight rain gear. I purchased a foldable one that ends up into a little package that you can wear around your waist from Mountain Sports I think. Also changes of hiking shoes or sneakers, in case you are out and it rains. I would say it stayed in the 50's to low 60's most of the week. You don't want the weather to stip your excursions. A hat is also nice. The idea of layering is good.
Good luck! Enjoy the glacier!