We will be on the Volendam's 7/20 back to back cruise to Alaska. As this will be our third cruise to Alaska, here are some ideas for you.
Consider renting a car in Skagway and driving up into the Canadian Yukon. The highway from Skagway to Carcross in the Yukon is an engineering marvel. Make sure that you have your passports and cruise pass with you when you cross the border. While this may not be as spectacular as the train, you will save lots of money and still see quite a lot of this fascinating place.
Ketchikan can be seen on foot. It is a very pleasant little city to walk around. This won't cost you any money.
Hope that this helps.