Make sure you stop at an ATM and bring plenty of Euros. Bring good walking shoes (not flip flops or open toed shoes). Unless you are a heavy drinker do not buy the open bar plan, we drink plenty and the drinks are reasonable and great quality.
The viking included tours are better than the add ons
If you use credit cards, be sure to place an international notification on the card before you leave. There are plenty of ATM machines available as you need cash during your trip. Be sure to check with your cell phone carrier to ensure you can use you phone overseas.
Don't over pack. We ended up with too many clothes.
Highlights of the Alps , an optional tour in Basel, Switzerland is somewhat overrated and very expensive. No need to pay the package drink, there's plenty of free wine, and beers during meal times, and no corkage fee when you bring it to the dining room. Wines and beer are dirt cheap in Europe.