I am a travel agent (semi-retired), and have been on many trips, visiting many countries, though only 2 cruises. I can seriously endorse river-cruising to be one of the best ways to travel and see a lot with very little stress at all. One of the best aspects of this mode of travel is the fact that you unpack once, and re-pack only when you are ready to head home. You can be as active, or as relaxed as you wish. After a few days of excursions every day, it's wonderful to be able to choose to stay aboard, spend a relaxing day, and do only what you wish to do, with no stress whatsoever. Our accommodations on board were modest, but more than adequate, the entertainment was good, the meals were great, with plenty of variety to satisfy all tastes, and all the excursions were very good. Best of all the service was outstanding. The staff went out of their way to be pleasant, kind, helpful, and do anything necessary to see that you were happy.
I use an Oxygen POC, and had a problem with my converter when I came aboard. I contacted the service desk, and they immediately came to the cabin with one that worked, and I was able to use it for the entire cruise, which eliminated any stress I would have had, and made my trip trouble-free!
We (My traveling companion and I) took the pre-cruise extension to Prague, and though the weather wasn't very good, it was still a very interesting side trip. We were booked into the Prague Hilton, and the hotel was very nice with pleasant, comfortable rooms, a gym, and a great buffet breakfast every day. The excursions were interesting also, and the downtown area of Prague is great for exploring.
Taken all together, it was a great vacation, and I would highly recommend Viking Cruise lines for this wonderful, close-up view of European life along the waterways.