For river cruise companies, I'd go with AMAWaterways, Avalon, Scenic, or Uniworld. Viking will typically have 190-210 people on their ships, while these lines tend to be around 140-180, depending on the ship and time of year. So more space for each individual passenger. More opportunity to be adaptable. For example if you are active, AMAWaterways offers active excursions including longer bike rides and hikes in many ports. At the same time, I've had them run gentle walking excursions just for me. You'll also have more things included vs. Viking.