The most popular stops for any cruise line are Ketchikan, Juneau and Skagway and some roundtrips hit Sitka. There are a couple odd ports other than this that some cruise lines stop at.
Ketchikan is mostly known for flight tours, totem parks, kayaking, fishing and if the time is right salmon runs. Ketchikan is also a rain forest so chances are good you will get rained on here.
Juneau is mostly known for tram ride, Mendenhall Glacier, whale watching, flight tours, raft rides down the river and panning for gold.
Skagway is mostly known for flight tours, its Gold Rush history, trips into the Yukon and the White Pass Railway.
All of these ports also have other tours and interests but this is what usually comes to mind when thinking of these ports.
Some of the best tours of course are the most expensive. Helicopter tours that land on a glacier are a must do if you can afford it. A whale watch is also one of the must do's if you can afford it. Most tours in Alaska regardless of the cost can be interesting from the beauty of the country or the historical knowledge you can gain.
As far as one way or roundtrip tours go I personally prefer to go one way over a roundtrip. You will see many of the same things with a roundtrip, however I feel a roundtrip misses the best glaciers and the best sites from the ship because it turns back to soon. Roundtrip is cheaper due to air generally but it still misses some of the best sites.
If you are doing a one way that includes a land package I would start in the north and head southbound because the land portion can wear you out and you can rest on the cruise. If you are just doing a one way with no land I would go from Vancouver to Ankorage for the reason that things get far more beautiful as you head north.
I have cruised Alaska many times and will do so many more times. The only thing I have done twice in Alaska is a whale watch because there is so many things to do and see. I am just a fanatic on whale watching so that is why I always do that at least once every cruise.
