My family and I went on an Alaskan cruise last week. It was my first cruise. I liked it. There is a lot you can do on a ship during a cruise. There is a casino, Broadway-style shows, art auctions, night clubs, various games (like trivia), and much more. I didn't do most of it, yet I wasn't bored. Just sitting on the balcony reading a book and watching the islands pass by was quite enjoyable. The food was amazing. Every evening is a five-course meal. The staff is amazingly helpful. Alaska is has a beautiful coastline (when the fog isn't in the way). Our cruise went to the Inside Passage, Juneau, Skagway, and Ketchican. I liked Skagway the best even though it is the smallest. It is an interesting town to explore.
I can now see why there are so many special interest cruises. There is a lot of time hanging around--especially in the evenings--that it would be really cool to walk by a lounge and be able to join in a conversation on something of interest to you.
If you've never taken a cruise, consider it. It's definitely worth doing at least once.