There are plenty of cruises of less than 9 days. The beauty of a cruise is you can go back to your cabin and have a nap, or read a book, or work on your computer, or ...
When we did an Alaska cruise 10 years ago in June, I was walking on the Promenade Deck one evening after dinner and not far away I spotted a Beluga Whale. He swam nearby for a while and then was gone. We spotted bears along the way, although we had to use our binoculars. And the sound a glacier makes when calving is just amazing in person. This was one of my favorite cruises.
Here are some with fewer days, just in case you reconsider:
http://cruises.united.com/results.do...CruiseId=false