After sailing on the Forseti, I can highly recommend a river cruise. There is no comparison to be made with an ocean cruise. It is a unique vacation and a fun way to see some sights in a much smaller environment; our ship had only 190 passengers and 50 crew. Just be prepared for really small cabins and bathrooms unless you book the penthouse!
There are numerous companies to choose from, but we were pleased with Viking and I would sail with them again. In my opinion, the company provided a classy, quality vacation designed for mature travelers, which we certainly are. The service was very, very good. Indeed, "no" does not appear to be in a Viking crew's vocabulary.
We also did both the pre-cruise tour thru the Loire Valley and the post-cruise tour of Paris. I would do the Loire Valley again, but we could just as easily have done Paris on our own.
We were traveling with a large group and planned to spend most of our time socializing in public areas, so we booked a standard cabin. It was really small, and the bathroom was so tiny it discourages a second trip to the breakfast buffet. However, it was spotlessly clean, well appointed, and comfortable for our "sleeping and showering" purposes. If my spouse and I were traveling alone in the future, we would book a larger cabin with a balcony.