Welcome to cruise-chat! Three years ago we did the same land/cruise tour that you are doing. What a wonderful experience! The next year we did the cruise only. Both trips were fantastic, but the land tour is really special because you get to see the interior of Alaska and enjoy the wildlife there.
As Sandy said, each port offers a huge variety of excursions. In the opinion of many people who have been to Skagway, the White Pass Railway is a "must do". We took the RR to the top of the mountain and the bus down. The bus stopped at Liarsville and it was a fun experience. The train ride is really, really nice and I would highly recommend it.
Whatever excursions you choose to do, seeing Alaska is an awesome experience.