I have been on 3 Uniworld cruises: Rhine, Seine and Rhone. We are traveling the Danube this September. I believe all ships had a small gym with a treadmill, bike, some dumbbells at a minimum.
For a first cruise, the benefit of the Rhine (our first cruise) and the Danube is that you get to travel through several countries with little effort (you unpack once, hopefully) and planning. They are major rivers with a lot going on. The villages along the Rhine were right out of a story book.
The Seine and Rhone rivers were more quiet. I loved our Burgundy/Provence cruise last year. We traveled with many Covid restrictions in place so it was more quiet than usual. We started in Lyon, known for its food, Roman ruins and the French Resistance. All the towns along the Rhone and Saone were charming with endless opportunities to drink rose and other wines. Arles was one of our favorites with more Roman ruins and the site of many Van Gogh paintings. After our trip we took a train on our own from Avignon to Nice which was wonderful.
I enjoyed our Seine cruise since it was my first visit to Paris. However, if you have already visited Paris, Giverny and Versailles, it might not be your choice for a first cruise. Another thing to consider is that several of the excursions involved long bus trips such as the beaches of Normandy and Mont Saint Michel. I felt it was less about the river and more about the excursions, which were wonderful.
All our cruises have been in September. The percentage of men wearing jackets for the captain's dinner and possibly the last night was about 33% on average, but more on the Rhone cruise.