DH and I went toward the end of May. Luckily, we experienced good weather - some rain in Juneau (expected) and cloudy some of the other days. The week before was a lot colder and more rainy. Actually, cloudy days are good if you are visiting glaciers. The colors within the glaciers are more prominent.
As for excursions from the ship -
Skagway - we did the dog-sledding on a glacier in the morning. If I remember correctly, the afternoon glacier tours tend to get cancelled more because of bad weather moving in. We had a great time on this excursion, glad we did it but it was a very expensive excursion. I think if we would have arranged this outside of the cruise line we could have saved some $$.
That afternoon, we did the train ride up the mountain. We really enjoyed this excursion as well.
Juneau - a little rainy. We did the wildlife excursion in the am. While others seemed to have had a great experience, we DID NOT. I don't know if it was due to the time of year we went or just a bad day. We almost did not see any whales. I think the tour operator was worried that he was going to have to refund our money (you are guaranteed to see whales). It wasn't until the end of the tour that we finally saw a couple of humpbacks. (Too early in the season for orcas) We could barely tell anything about these whales as the only thing we saw was the hump on them. We were very disappointed.
That afternoon, we went to Mendenhall glacier. We enjoyed our visit there.
Ketchikan - we went on a fishing excursion. (Not salmon, too early in the season). There were several people doing this excursion but every couple or maybe every four people had their own guide. We went out in small fishing boats, caught some fish and then our guides took us to a location off the beaten path where they cleaned and cooked all the fish that everyone caught. Along with the fish, the meal included other delicious items. This was fun. Saw LOTS of eagles and other wildlife around our campsite. In addition, our guide - after hearing about our almost-failed wildlife excursion in Junuea - took us on a little detour while we headed back to the tour office. It was on this tour that we saw several humpbacks and other "critters". Some of these humpbacks got a little too close to our small little boat....or so I thought. I think our guide got a little nervous as well at one point....hard to tell.
I really enjoyed this excursion. Our guide was quite humorous. However, it was quite pricey if I remember right.
As for tips - two things stand out (and I'm sure there are other things I'll remember soon):
1) Take some duct tape. Yes, you read correctly. There are travel sizes now.

I've read about this on the cruisecritic board. I now take it on just about every vacation. And, we needed it in Alaska. The airline just about destroyed a piece of our luggage. We had to use the duct tape to hold it together. This luggage was not cheap and rated very highly so I'm not sure what the airline did to get it in the condition it came out in. I'm just thankful my DH's underwear didn't spill out all over the luggage conveyor belt at baggage claim.
Also, I've heard of people using duct tape to hold curtains back or together, holding the balcony doors open, etc.
2) Extension cord. Again, this was a suggestion on the cruise board and we were glad we had it. However, I can't remember exactly why we needed it.

(I'm losing my memory). I think it might have had something to do with there not being an electrical outlet by the dressing table and I needed it for my curling iron. The Diamond Princess, however, may not have this issue. We might have used it for DH's sound machine as well. I do remember being glad we brought it.
Ok - a third tip:
3) Highlighter pen - You get a daily newsletter outlining the various activities on the ship for that day. Use the highlighter to mark the activities you want to do. It's easier to read than a regular pen/pencil.
I'm sure I'll think of other things. As one poster said, go to cruisecritic.com for a wealth of information.