As far as seeing vendors at the port, the most noticeable place for us was at Ketchikan. We did walk around a bit in Skagway, and there were probably a few vendors there but I can't say for sure what they were offering. For Juneau, we left directly from the ship on our excursion, so I didn't see any vendors there although I'm sure they were somewhere. They were definitely in Ketchikan though because we almost booked something with one of them before our Misty Fjords excursion but were nervous because of the time.
As for Mendenhall Glacier, we didn't run at all. I did hear some huffing and puffing behind me on the way back because I was so nervous we weren't going to make it in time to catch the bus. I walked pretty fast and figured the rest had best keep up or get left at Mendenhall Glacier.
It's doable if you get right off of the bus, go straight to Nugget Falls, don't mess around too long, and head straight back. Hopefully, by the time you'd do this, they will figure out that guests need more than 45 minutes! It's neat even if you don't get to walk down to the falls, but we were determined because we love waterfalls!
I normally wear capris a lot, and honestly, I only wore jeans on our cruise. It was nice for the most part, but I'd say it was jeans weather. However, it might be way different when you sail on July 5th. And, I'm kind of wimpy. I don't like to be cold. I've read other posters on my same cruise that said they wore short sleeved shirts, etc., and I secretly thought they must be insane. It wasn't that warm. In fact, our sea days were downright cold with the exception of Tracy Arm day. There were quite a few people in the pools at different times. The pools are heated to between 88-90 degrees, so I'm sure they are fine while you are in them. It would be the getting out part that would not be okay with me.
Good luck and happy sailing! You have to be getting so excited! Your trip is almost here!