You will have a wonderful time in Alaska. Golden Princess is a great ship. Be aware that she is BIG. There is a LOT of walking from one end of the ship to the other! There is a nice gym and I know they will be delighted to see your dad there!
There are so many things to do on Golden. You can participate in the activities from trivia to craft classes to ice carving demonstrations, or you can just find a quiet spot to sit with a book and watch for whales out the window. At night there are big production shows and small lounges with music. Whatever the tastes of your family there is something to do.
There are also many many shore excursions in Alaska. Some are very active, some are more sedate. I have made a page of some of them that we have enjoyed at Alaska Excursions Guide
to give you an idea.
From the description of your family group, I would seriously recommend that you consider renting a van (or two cars) from Avis in Skagway and driving over the White Pass yourself. The railroad is interesting if you are a railroad buff, but driving yourself gives you a lot of freedom and the ability to stop when you want to. Check my page at Driving from Skagway into the Yukon
I know you will have a GREAT cruise!