DON'T ever turn them back in......You will get a terrible exchange, either save them for your next trip.....OR if you are on a cruise, use them for extra tips for the service.
We just walked from the end of the cars to the Glacier in July....Layers, we had a very warm day, lots of people in shorts....It is a walk that takes about 40 minutes from the end where the Trollcars stop. Need good walking shoes.
While we have stayed other places I like better, The Marriott is by the museums, plenty of restaurants, has self laundry...which did come in handy...and has a wonderful breakfast.
Much as I love Viking...over the past few years their payment policy has become..well odious....Explore other options, other lines.....Viking does this because they can.