It sounds as if you're thinking of the round trip Inside Passage cruise and not a one-way to/from the Anchorage area.
IMHO...two words...Glacier Bay. For a first Alaskan cruise you must see Glacier bay. Save Hubbard for a future cruise. It's beautiful but not like Glacier Bay. This will reduce your choice of cruiselines as well. I believe that of the mainstream lines Holland America, Princess, and Norwegian are the only lines that go there. And not every one of their ships go. RCCL and Carnival might but I'm not sure. Celebrity doesn't.
We've been 3 times and there were park rangers brought aboard while in Glacier bay and Hubbard.
If you can do it I would also suggest the roundtrip to/from Vancouver instead of Seattle. That route is more scenic because it goes inside of Vancouver island and winds through more tight channels. That's been our experience at least.
From the ships you will certainly see a lot of wildlife. We saw Humpback whales, Dahl porpoise, and Orca although from not very close. Bald Eagles are everywhere although better seen from the towns. From the ships they look like white dots in the trees.
We've been there in August and the salmon were running. There is a salmon ladder about a 5 minute walk from the ships in Ketchikan.
There are many options of what to do while in port as well. A lot of that will depend on budget and likes/dislikes. I would suggest at the very least to go to Mendenhall Glacier while in Juneau. 10 bucks round trip from the dock on a bus. Extra charge for the visitor center (have you figured out by now that I love glaciers?).
If the weather is clear take the sky tram to Mt. Roberts in Juneau as well. Spectacular views and this also is right at the docks.
In Skagway the White Pass Rail is great. Wonderful scenery and great history. Some people are bored with it however and it's a bit pricey. We felt it was worth it.
In Ketchikan the Duck tour was fun....very silly...but fun. And you get a good idea of the area. Go up the funicular and walk down the married mans trail. Scenic and easy to do.
In all of these towns, and Sitka as well if you go there, are easy to walk around and shop and enjoy the area. The ships (that don't tender) dock pretty much in the middle of the towns. We really enjoy doing our own walking tours.
Enjoy your cruise. I'm sure you'll love it as much as we do.
