Jules Verne does wonderful river cruises on converted barges. They hold a maximum of 20 passengers, but in my experience there may be fewer. On my last trip we were 7!
Cabins are small but ok, with big windows,and in my experience the crew are excellent. I have been to Alsace, Lorraine and the South of France. Catering is excellent and drinks are included. Shore excursions each day are always interesting. There is the option of walking or cycling along one some of the tow paths. Each barge has a hot tub, so take swimming costume.