We went on an Alaskan cruise as part of our honeymoon a few years ago, have to say, absolutely loved it, I wasn't too optimistic as it was something wifey wanted to to do more then me, but it was great
Firstly, get a cabin with a balcony, yes you'll pay more but it is less claustrophobic and sailing up Glacier Bay, sitting on the balcony wrapped up warm with a mug of coffee and seeing a couple of whales as well is a memory I'll have for a long time
You don't need to do an excursion as a port of call, sometimes you are better to go wandering yourself. We did that in Juneau and it was great, Skagway was a bit of a dissappointment, we did a train trip up where the gold prospectors went which was great, but the town itself is a total tourist trap. Ketchikan was very good but Wrangell was my favourite, there's nothing really there but it was very untouristy and because of that, you got a better picture of what it might be like to actually live in Alaska, certainly wandering around the local clothing/gun shop was not something you'd see in Grafton Street
Problem with a cruise is that it will give you a taste for Alaska, but you'll want to go back for more.
Spend a few days in Vancouver as well, great city