First, I'll tell you I'm not a big supporter of cruise line's excursions - too costly, too controlled, and too crowded. We much prefer to either do our own thing or to do independent excursions. There are several companies to look at including Shore Trips.
Having done a 14-day cruisetour and only doing excursions with independent companies, I can tell you first hand we were so glad we did the several independent tours we did. For example; we did the helicopter/dog sled tour in Juneau. The cruise line wanted $100 more per person than Shore Trips. And our helicopter only had another couple along with us, so it was not crowded, plus they flew over a couple of different glaciers giving us a longer flight than those operated for the cruise line.
Princess is noted for being a bit more expensive than independents. We were in Aruba and wanted to do the 4-1/2 hour tour around the island on Harley's. Princess wanted $355 for the tour and we got it directly through the Harley dealer for $155 and it was exactly the same tour!
I could give you several examples, but trust me when I say there are definitely cheaper, easier, and less crowded ways to do the excursions than through the cruise line.
Now, with that said, the most popular excursions do tend to sell out, so it's often better to book beforehand. And yes, often times cruise lines will offer tours on the ship that are not offered on the website because things are always changing.
But overall, you will definitely enjoy your cruise to Alaska. It truly is a wonderful place to visit.