Hi wwide, and welcome to Cruise Chat.
September is a good time to go to Alaska. The weather will likely not be significantly colder than August, but the prices in September are lower. The towns in which you stop will be less crowded, as population often increases several fold in summer as folks from the lower 48 drive up to Alaska and stay all summer.
Going northbound or southbound (the more popular direction) is great especially if you're combining the cruise with a land tour before or after your cruise. If not, the cruises that go round trip from Vancouver or Seattle are still hitting the major ports without the need to fly to or from Anchorage/Seward etc.
Excursions are often hard to recommend without knowing your preferences, activity level and budget. Flightseeing is amazing but expensive. Doing one of the dogsledding adventures is totally unique but very expensive. For general sightseeing for a first time visitor, the Skagway and whale watching tours that were mentioned are terrific. Last time I was in Alaska, we did kayaking in Juneau, climbed the Chilcoot Trail in Skagway and hiked a rainforest in Ketchikan. You're right that the excursions all sound terrific, because they are!
I agree with waterart, too, that seeing any of the glaciers is an awesome experience. Don't spend too much time wondering which glacier, because it's always possible that conditions during your particular cruise can keep the ship from going to one, or allow the ship to get much closer than usual to another.
I assume you've seen information like this
on Icy Strait Point. Perhaps someone who has been there can give you more information. If you put Icy Strait into the "Find" function near the top of this window, you'll see several previous message threads with information about that stop.