Leaving from Seattle is basically just a "different" cruise. To leave and return to Seattle the ships don't get as far north as the north bound cruises leaving from Vancover. But you would still see the common ports.
The favorite activities for us were the whale watching, snorkeling (they give you a full body wet suit to keep you warm), and then it is a toss up between the kayaking wildlife adventure and gold mining. One tour we were dissapointed in was the river rafting trip. It took most of the day up and we got to see some Eagles, but mostly we floated on a river (calmly) so it was a bust in our minds. We could have done something else with the time.
Just be prepared for the extra cost of Alaskan excursions versus the caribbean cruise excursions. The Alaska tours cost about 3 times what typical Caribbean ones do. If you want to take that helicopter ride, be prepared for several hundred dollars per person.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007