My wife and I thoroughly enjoyed Viking's Rhine Getaway on the Viking Sun in mid October.
The start wasn't too auspicious because the Rhine was running high and we had to embark from Breisach instead of Basel as expected. Turns out that this was, to me,value added for the cruise because we got to spend extra time there. Although the weather was rainy, the town was delightful. I can highly recommend the Baden wines available in the wine shop on the town square.
Breisach is the stop for the first of the tours offered as part of the cruise. We chose to skip the included Black Forest tour because we expected it to be too commercial based on experience with similar tours in Germany. The optional (extra cost) Colmar tour was the only disappointment among the tours we experienced. The guide was very uninformed abut Alsatian history. The included Strasbourg tour, on the other hand, provided an excellent initiation to Alsatian history and culture.
All of the other included tours were all very good. Two standouts are the Marksburg Castle and the walking tour of Cologne.
The highlight of the Rhine Getaway is cruising the Middle Rhine from Braubach to Koblenz. As I mentioned earlier, it was rainy at the start of the cruise. By the
time we reached Braubach, the weather had cleared. We had a spectacular fall morning to view castles from a river perspective. The commentary over the ship's PA system added to our enjoyment.
In summary, while weather can be risky, October cruising has real advantages. In particular, crowding, both on the river and on land was minimal. Cost is also lower than the height of summer.