The weather in Alaska varies greately, we went the end of August, first week in Sept. and we did have days in the 70's, which is a bit unusual for that time of year. I did pack a pair of shorts and wore them. Most days though, you'll need pants, t-shirts/sweat shirts so you can layer and then as the day warms, take things off.
Here is a link to the 2004 Alaska planning guide, you should be able to get lots of tips on packing, excursions, etc.:
2004 Alaska Planning Guide
What ship are you cruisin on in Alaska?
We cruised on Celebrity Infinity in 2004, here is a link to the review I wrote:
Celebrity Infinity in Alaska