I've just written an article on River cruising, I'm just awaiting its approval by the Just Luxe team. River Cruising is ideal for senior citizens, they are its main target audience. The beauty of it is that you are never far from the shore (compared to an ocean cruise) so the scenery is constantly changing. Going with a company like Viking River Cruises or AMA Waterways means you get decent luxury accommodation and food (and with less than 200 people on board, food is comparable to a high end restaurant, with locally sourced produce; unlike on an ocean cruise where it can be a bit like feeding an army).
Have a look at http://www.rivervoyages.com/river-cruising/ which gives you a bit of an overview of what to expect.
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