To help you get the best Alaska cruise possible, here are some basic facts about the options that are available:
Most Alaska cruises are 7 nights, but there are more every year that are longer, and you can tie 2 or more cruises together to make them any length you want. Some years (not every year) there are a very small number of cruises that are shorter than 7 nights, but you see very little of Alaska on those (normally just Ketchikan).
Some Alaska cruises are advertised as Inside Passage cruises, some aren't, but almost all Alaska cruise ports are in the Inside Passage so in reality every Alaska cruise is an Inside Passage cruise.
You can go one-way, or return to the starting port - usually Seattle or Vancouver, but San Francisco is available as well. One-ways are the way to go if you want to explore more of Alaska (such as Denali National Park) by motorcoach, train, RV or rental car, but the air to or from Alaska can add considerably to your total cost.
For those prone to seasickness, Vancouver departures offer more sheltered waters, as they sail to the east of Vancouver Island whereas Seattle departures sail to the west of the island in the open ocean. For the same reason, Vancouver departures offer more hours of scenic sailing.
Is seeing glaciers a priority for you? If so, you want cruises that visit Glacier Bay, Hubbard Glacier or College Fjord. Tracy Arm is often considered as a glacier viewing day but very often ice in the fjord prevents the large ships from getting within sight of the Sawyer glaciers.
If you want to see wildlife in particular, budget for quality excursions during your port stops. You see little or no wildlife from the ships - while whales are usually seen if you spend enough time on deck, it takes a whale watching tour from Juneau or Hoonah (Icy Strait Point) to get up close and spend time with the magnificent creatures.
All Alaska cruise ports are very walking-friendly, and Juneau and Skagway in particular have excellent hiking trails very close to the cruise docks.
There are ships of all sizes sailing Alaska now, from the very small luxury ships of American Safari to the mid-size luxury ships of Regent Seven Seas, to the 2,000+ passenger ships of the major lines. Smaller ships get you closer to nature, at a higher cost. Here again, your priorities and budget will be the prime factors in deciding which is right for you.
I sail Alaska fairly often, usually working as either the ship naturalist or Destination Speaker, and questions about any aspect of Alaska cruising are welcome here. I've sailed Alaska with Royal Caribbean, Celebrity, Holland America and Princess so far.