I agree with going to Glacier Bay ... it will be a day you won't soon forget.
We have gone in every month of the 'Alaska Season' except for April and October.
In April and May, you will run the chance of not being able to get all the way down the Fjord to view the Glacier (not sure if this is true for Glacier Bay). Our friends went in late June and had perfect weather in every port.
We had excellent weather at the end of July. We also had a mixture of cool and warm weather but no rain several times in late August and early September but then very cold, rainy and windy weather in late September.
I'd shoot for July / August for a better chance of good weather but if you are looking for a bargain, late August and early September if you are looking for bargain rates. And .. there should be no issues with getting up close to the glacier. If at all possible, Alaska is the place you want to get a balcony.
Also, you might find a flight to Seattle a lot cheaper than flying into Vancouver. We try to always go out of San Francisco, 3 more days of cruising with that itinerary. They have plenty of events, lectures, activities on the ship related to Alaska while onboard.