I'll second WildAlaska's caution about clothing, especially if you camp out. I backpacked for 3 weeks in Alaska one summer years ago (Glacier Bay, Denali and Wrangell St Elias, 1 week each). It rained every day but one, and the average daytime highs (in late July and early August) were in the high 40's and low 50s, and we even got some snow (better than cold rain).
We were fine, but we had GoreTex and poly fleece underneath to keep us reasonably warm and dry for the trip. The weather sucked, but it was beautiful up there! We saw many large critters, up close and personal as well.