After several longer river cruises (i.e., Budapest to Amsterdam) we thought that we might like what would be a much shorter trip (Douro river is only what, 125 miles long?) and kind of quirky itinerary; small towns and wineries with time to walk and explore.
Wow did we like it! Beautiful countryside in Fall colors, visits to often tiny towns, and so on. Castelo Rodrigo, for example, is a hilltop hamlet with about 48 residents. We had time on our own in some very small towns to just wander. Very restful, actually Quite different from say, Vienna!
I don't recommend this for one's "first cruise". Take a longer one, or several, with Viking and then try The Douro. Doing that really helped us notice the differences and fully appreciate the laid back leisurely nature of this trip. It is very well thought out and complete. I think you'll find it wonderful.
As always with Viking there are little extras; to be met back at the ship with a Port wine toast, local foods acquired almost every day, and some unique entertainments. Portuguese Fado music. for instance.
I HIGHLY recommend the post-river add-on trip to Santiago de Compostela in Galicia, Spain. The shrine of Saint James is not only the ending point of "The Way" pilgrimages, but also a UNESCO Heritage center. We loved the city and its foods. Tapas three times a day!
Our cabin was more than adequate; we are hardly there except to sleep and clean up!
Yes the ship is small; only about 105 passengers; you'll get to know almost everyone and everyone is Cool. Given the often narrow Douro River channels, the ship is perfect.
No fitness facilities on the ship, but you will walk A LOT!