There are so many things to do in Alaska that you'll probably have a hard time narrowing it down. There is a lot of great info on the Alaska board on the Destinations section of this site. By the way, NCL is not the best cruise line for Alaska. Best lines there are Princess and Holland. They get more permits into Glacier Bay because this is their area of specialty, and they are allowed to run tours into various areas of Alaska that the other lines are not allowed in. Your activities will also depend on your age and fitness level. We are in our early 40's and pretty active, so please keep that in mind when reading our recommendations.
That being said, my husband and I decided that we would do a whale watching trip in one port, a flightseeing in one port, float trip in one port, etc. So then we just had to figure out which port offered the best for our category. I highly recommend the whale watching in Victoria. Victoria is a lovely city (we did it pre-cruise, so we had more time), so I hope you can see that too. We were very lucky and had 3 pods of whales come together for feeding on the afternoon of our trip. I also recommend the Kodiak boats (like really hard rafts) because you're down so close to the water in them. We were so close that we could have touched the orcas! We also did a whale watching and wildlife boat trip out of Seward in the Kenai Fjords area, and I highly recommend that too because you see a glacier from the small boat down close to the water (it did a lot of calving while we were there, and it was awesome) as opposed to the much different perspective of seeing the glaciers from the height of a cruise ship, which is great too.
We saw bears in Denali and on one of our flightseeing trips and the same for eagles, so if a guaranteed sighting of either is important to you, you might want to consider that in planning your activities.
The Mendenhall Glacier helicopter flights that land on glaciers are really great, and they are out of Juneau. We also enjoyed the float plane ride out of Skagway to the Taku Lodge, but I think this is a trip offered by Princess only. If it's offered by your cruise line, it would be a great experience for you. The flight is very scenic, and you land at the lodge for a salmon bake. While the salmon is being grilled, you can hike or just enjoy the beauty of the lodge, which has a very interesting history and they tell you all about that while you eat. It faces a glacier, and the scenery is just wonderful.
We also did the Mendenhall Glacier float trip, which is a float with only a few rapids and is mainly for the scenery, and a whitewater rafting trip in Denali (mostly Class III rapids with a few Class IV) and can highly recommend both.
I also recommend that you take at least an hour or two to walk around all 3 towns. The Creek Street area in Ketchikan is a lot of fun, and Skagway is very picturesque. In Juneau, you can ride the Mt. Roberts Tramway to see the views of the harbor and the town. I think you should just wait and purchase your ticket (same price as the excursion) when you get to town in case it is cloudy that day. It's about $22, but you can use your ticket more than once during the day. The architecture in Ketchikan and Skagway is just great.
We also went kayaking in Ketchikan and horseback riding in Skagway. They were great trips. Many people enjoy the White Pass & Yukon railway, so that is also highly recommended. (We rode the train from Seward to Anchorage, Anchorage to Denali, and Denali to Fairbanks, so we decided to use our time elsewhere.) In Skagway, we also took the ferry (and did this independently because it's a lot cheaper than the cruise line's shore excursions) to Haines. It's a funky town that has incredible scenery in the harbor area.
I hope you will be able to go to Denali because it is so great, and I think you'll have a better time if you book that separately.
There are also bicycle tours and golf tours, so it really depends on your interests. We're great fans of the Frommers web site and Alaska book because we like to get good value for what we spend, so I recommend that you read up and then do your planning because there is so much to do in Alaska. A book like Frommers will also tell you which tours are better than others, which is very helpful because there's really not any bad scenery or activities in Alaska. I think you could cruise 10 times and not repeat your activities!
So good luck with your planning, and happy traveling.