Okay.........so i've worked on the cruise ships for the last ten years and here's some advice that i hope you will find helpful. I think by far the best company to cruise with is Royal Carribean (www.rccl.com) they seem to have the nicest ships with the best ammenities. Princess are a good second option. Holland America while it probably does have the most ships up there, for the most part isn't the best line to cruise with. Everyone i know who has either worked on them or cruised with them is very dissappointed with the service on board the ships. If your parents are new to cruising i would not recommend this line. Everything on the ship closes down about 11pm because of the age group of the passengers. While Holland America is nice, it definately targets pax (passengers) who are in the elderly group. The cheapest route to take would probably be Carnival Cruise lines who also have a few ships up there. Not for much longer i hear though.
Now you have to decide what cruise you want to take. You really have two options from Vancouver; you can take the 7 day cruise up to seward (anchorage) and fly back from there or vice versa, or you can take the 7 day cruise up to Glacier bay and back. Usually the main difference about these two cruises is, the return one goes to Sitka while the other goes up to Seward which is farther north and more "alaska like". While another post mentioned the land tours at seward I agree completely with him. I've done this and it was great. Get off the ship and five to seven days up near the top of Alaska. Or you can do the land trip first then take the cruise home. But the problem with this is you have to pay for a one way flight to or from Anchorage.
And the main cost saving advice i can give about cruising is to try to never book any of the land excursions you do through the cruise ship company or the ship. Try and have them planned out before you go and already have them booked and paid for. Some i believe you do have to do through the cruise ship company though (like the dogsledding on the glaciers) because of the limited space available. But there are tons of tour options you can do. There's a really nice train ride in Skagway, bear siteseeing in Ketchikan, whale watching in Juneau. Basically any type of nature related tour is available in Alaska. And usually about half the price if you can book it directly through the tour operators and not the Cruise ship companies. The best tour I've ever taken in Alaska was the dogsledding. Anyone can do it.. age doesn't really matter, not physically strenuous at all. You helicopter up to a glacier and get to go dogsledding for about four hours i think. It is quite pricey but if you can get it privately (rare but sometimes) it costs about 200 per person, otherwise i think it's between four and eight hundred.
Hope some of this helps. And hope your parents have fun. Anyone who cruises should do Alaska at least once. Very Picturesque

We went on the Seven Seas cruise line and had a blast in Alaska. They pretty much cater to you the whole time and schedule everything from helicopter tours, to dogsledding, to whale watching.

__________________
2006 BMW X5 4.8is Lemans Blue/Beige/Dark Maple
-Nav, Comfort Seats, Heated Steering Wheel,
Rear Airbags, AUX Input
2009 BMW M3 Space Grey/Black/Carbon
-all options