Just gave this advice to one of my clients going on an upcoming cruise to Alaska. I've been there seven times, so, been on a few tours.  Hope this can help! 

When talking about shore excursions a lot really depends on what you personally like to do.  I did a bike ride to a brewery and to see the Mendenhall Glacier in Juneau and I loved it so much I did it another time. Juneau is all about helicopter tours, dog sledding, glacier hikes, etc.  You should sign up to see the Mendenhall glacier and as a back up, if you want to spend that much money, book a helicopter tour.  They often get cancelled due to the weather, so, don't be too disappointed if it happens.
 I've booked five of them and have actually been on only ONE.  In Ketchikan the tour I loved best was to Misty Fjords on a float plane to a bear watching tour and Salmon hatchery.  So very cool. Also, you have to see the Lumberjack show. It's so much fun and the guys are so sexy! 

Sitka is pretty quiet.  I don't really know it, but, maybe whale watching there.  

You can watch whales from the ship, but, they're not guaranteed and are often just a fleck miles and miles away.  Best to do a specific whale watching tour that will get you close up to them. It's amazing to see whales close up.  Same with bald eagles, bears, etc.

The last stop, Victoria, is home to the amazing Butchart Gardens. If you could get a tour there while it's still light out that would be nice. They have night tours, but, you miss so much when it's not lit up by the sun. They have a nice downtown area and the Empress Hotel is worth a look, and, if they have afternoon tea it's quite a gay time.