First thing I would recommend is to spend a day or two in Vancouver before (or after) your cruise. It's such a beautiful city with so much to see. I love the city and hope to go back. You have Gastown right near the port, you have Stanley Park, there's an Anglican cathedral, you have an hour drive up to Whistler, which is gorgeous.
Here's the steam powered clock in Gastown that chimes on the quarter hour.
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Definitely bring clothes that you can layer. We went over July 4 and we had a high of about 70 degrees, but it got much colder in the afternoons/evenings or when we got close to Tracy Arm glacier. Also, nobody tells you this until you get there, but Alaska is part of the only temperate rain forest in the northern hemisphere. Yeah. . . rain forest. So pack a travel umbrella.
But also pack your bathing suit. Remember, the pools are heated to 85 degrees.
Keep an eye out for unique character photo ops. Brother Bear characters were on our sailing; Minnie and Mickey had great "cold weather" costumes.
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Our favorite excursion was a bike ride to a glacial plain in Skagway. Skagway was also my favorite port, personally.
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We also did a whale watch in Juneau. It turned out to be very disappointing. Be aware that your experience can vary widely with a whale watch. Our first trip to Alaska we had a GREAT one, with whales coming up to our boat; but on our second trip we saw very little wildlife. So I would still recommend going, because if you get a good one, it's an amazing experience. But be aware that it's possible to get a less-than-good one.
Here are a couple more pictures from Skagway:
This is a railroad snowplow! They put it at the front of the train and it throws snow off the tracks. I thought this was really cool.
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One of the earliest billboards in Skagway, painted onto the mountain wall:
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