Just to clarify a few things that we disagree with that some of you mentioned above - as we have been river cruising since 2013 and we are currently in our mid 50s. We don't consider ourselves old or unfit. We are very well traveled - on our own, on river cruises, on land tours with Rick Steves tours, land tours with Tauck, etc.
River cruises are NOT only for old people who aren't 'fit.'
We find the cruises we take have VERY active excursions to choose from. You can take biking excursions, hiking, castles, cathedrals, wineries, Abbeys - all types of fitness levels, experiences, etc.
We typically go with Tauck now, there are NO umbrella/flag carrying guides. We have been on all the lines - Viking, AMA, Avalon, Uniworld, Scenic, Gate1.
We love the fact that we can unpack once and still travel and explore and see lots of areas in 1-2 weeks on a RiverCruise. We don't have to deal with hotels, transportation, tickets for places, etc -
Now we do tend to extend pre and post cruise, so we do end up on some trains and hotels on our own - but we enJOY that aspect too.
As far as the Xmas Markets at night on a RiverCruise . . . it is true that you are typically back on the boat for dinner and then cruising. BUT - it does start getting dark over there in Dec between 3-4, so we do see the lights come on. Also - while the markets are stunning at night, the shopping and 'seeing things' are MUCH easier during the daylight hours. We always extend 3-4 days PreCruise and 5-6 days PostCruise, so we have ample time to be in the Xmas Markets at night. When we spend the money to fly to Europe, we always stay a minimum of 2 weeks, so if you do a 7 night Xmas Market Cruise - I encourage you to add on another week and see some markets on your own and enJOY the nighttime lights, etc.
As far as the Xmas Markets are only for those who 'accumulate things' - while you can buy things, you certainly don't have to. We enJOY the ambiance, concerts, cathedrals, palaces, abbeys, FOOD, Gluwein, people, decorations, and the towns. The markets go back centuries- they are quite an amazing experience. We go every other year and never tire of them.