You will love the photo opportunities on your Alaska cruise - awesome.
I have done an Alaskan cruise twice. Once was the Seattle route you are doing.
As Sandy mentioned go prepared for rain in Juneau. It's a rare event for it not to be raining.
I highly recommend the helicopter/glacier trek.
Be sure to save some time to visit the Red Dog Saloon. IF it looks to be a clear day the tram ride downtown is nice for photos too.
I also did the salmon hatchery/glacier tour on land - helicopter MUCH better.
Skagway for me was the White Rail Pass Railway.
Along with time to wander downtown.
I have loads of photos in my webshots album. Take a look......
As for a tux - really not required. A suit is fine.
Layering for daytime clothes is key.
ENJOY your cruise.