Cruised there last year.
We went out of Vancouver just because that’s where the ship happened to leave from. I’m glad we did...we spent four days in Vancouver and loved the place.
If you take a round-trip cruise, you will be in Canada one way or another. If you take the cruise from Seattle, it must stop in Canada to satisfy the Jones Act. If you take the cruise from Vancouver, obviously, there is no such issue. Generally, the cruises from Vancouver will be seven nights, the cruises from Seattle will be eight nights.
Most of the week-long round-trip cruises from Vancouver will hit Skagway, Juneau, Ketchikan and will likely spend a sea day in a fjord idling next to a glacier. If it’s an eight day cruise, it will likely go to Glacier Bay as well. The round-trip eight-day from Seattle will do the same, but add a stop in Victoria, Canada on the way back.
(The fjord will usually be Tracy Arm or Endicott....Tracy Arm is usually mentioned in the itinerary, but weather or ship traffic can result in an audible and Endicott is usually plan B.)
The one-way cruises will usually involve a flight to or from Anchorage. Given that it’s a one-way flight, it will be pricey. You could also look into a land and sea package - I know Princess and Norwegian do them. (E.g. You fly into Anchorage, they’ll take you by rail to Denali, tour it for a day or two and then they will start moving you towards Seward. You get the ship there and then hit Hubbard, Icy Strait, a glacier somewhere, Skagway, Juneau, Ketchikan and eventually wind up in Vancouver. Or you do it in reverse.)
I don’t know what I would say is a “must see” - it’s all overwhelming. You spend a lot of time closer to shore than you would be on a normal cruise, so you see quite a bit. If you like to hike, I have a good one for Skagway, but we did some touristy stuff in Juneau and Ketchikan and enjoyed it.
My most immediate advice is....hurry up. If you want to travel in July, you’ll need to get a move on. (Note...prices tend to get much cheaper towards the middle of August into early September.)