Just thought I might as well add a few of my thoughts to add some more detail to the thread...

The wife and I did this Alaska cruise itinerary for the first time last year (2009). Point to now that is that we're from the Pacific Northwest so we didn't find a lot of the sightseeing options unique/or interest.
Skagway
We did a tour (found it based on reviews in the ports of call forum in Cruise Critic) that started with a one way ride on the White Pass railway, minibus ride to Carcross and back. We then just toured Skagway for the rest of our stay.
I'm not a railway buff but the ride was fun. You got stand outside between the cars and the views were nice. The drive to Carcross was nice because of the views: nice lakes reflecting the background mountains, a couple of bear sightings, etc. We stopped at a dog sledding complex that was expanded/configured to serve as a tour destination. (It was ok.) The price was reasonable (and we generally hate paying for the convenience of a tour, preferring to DIY as much as possible). The only thing that we kind of disliked was that the guide's (who was really helpful during the tour taking pictures, stopping for pictures, etc) stories/commentary got a bit preachy/sappy at the end which we felt was to try to solicit more gratuities (which we were going to tip well already because he was so helpful but this bit really turned us off).
Skagway itself was pretty quaint. Easy to walk through with a limited amount of time IMO.

Juneau
The initial commercial area that you walk through is pretty touristy. It reminded me of other cruise stops we experienced in the Caribbean with a crapload of stores selling tourist trinkets.
We grabbed a free map at the state TI (beware of the "fake" TI's that are just a front for tour companies) and did a walking tour of the neighbourhoods, walking by the governor's mansion, legislature, etc. Nice walk. We then hiked it up Mt Roberts. It's quite doable if you're in ok shape. However, we went early in the season so we encountered snow on the trail near the top. It's quite the tourist trap on top of the mountain IMO and I don't think it's worth paying to ride the tram up.
After lunch (at a restaurant at the base, tho I wish we found Tracy's Crab Shack first) we just hopped on a tour bus to the glacier. Because of all the buses, you don't need to make a reservation on the ship IMO. That said, we found the glacier a bit of a disappointment because: You can't go up to the glacier (compared to being able to walk on the glacier in the Columbia Ice fields) and the Hubbard glacier we saw later in the cruise was more spectacular.

Ketchikan
We didn't do any tours; rather just walked around town... Creek Street, rode the short $2 funicular, walked some trails, etc. We didn't find enough things to keep us occupied walking around town so an activity might have been good for us in Ketchikan. We didn't see the lumber show because we can see them here and there during different events in Vancouver (seen one, seen them all kind of thing for us).