Lucky you! We thought we would only go to Alaska once, also. However, we were so enthralled and amazed by it's beauty that we went again. I am sure another Alaska cruise is in our future!
The one excursion that I believe is a must is the White Pass Railway in Skagway. We took the train up the mountain and a bus back down. The bus stopped in Liarsville where we had a delicious grilled salmon dinner.
There are so many interesting things to do at the other ports that it's hard to narrow down. In Juneau we took a floatplane to Taku Glacier Lodge. That was a really neat experience. The second time we were in Juneau we took an excursion to the Mendenhall Glacier, another worthwhile experience.
I'm sure other people on this forum will be glad to offer more ideas.
Be prepared for some of the most beautiful scenery you will ever see!