We enjoy Grand Circle. Their cruises are typically longer duration (~2weeks) and the price point is comfortable for us.
They break the cruisers into several groups, each with Program Director, that you stay with for the entire cruise. The included tours utilize local guides and the PD stays with the group. Frequently the PDs offer additional tours/activities during the afternoon or, if a late departure, after dinner.
GCT provides future cruise credits (~5% of trip cost) after 2 or 3 trips. It increases to ~7% after more tours (8-10?). All trips with GCT/OAT count, not just river cruises. They also offer a discount on multiple trips in one year.
As we enjoyed our first trip an as GCT/OAT also offer land tours and small ship cruises, we kept coming back.