Yes, staying on the boat is always an option. My dad is the same age and he stayed on the boat most mornings. Then we would come back to the boat for lunch and he would join us on the afternoon excursion. It let him sleep in and also avoid some of the longer walking tours. I'd say out of 70 guests, there were usually 10-15 people that stayed on the boat at any given time. The room TV has a lot of English programming, including some on demand movies and TV shows you will recognize. There is a small pool, a sundeck, a lounge with occasional live piano music, a small library. Food and drink is always available. Free WIFI. There should be enough to keep her busy for a few hours while you do something else.
There are some obvious strenuous activities, like two different bike rides, but don't underestimate the amount of walking you will do on some of the city tours. And there are some elevation changes and uneven ground, so someone older may want to rest their legs from time to time during the trip. Also, you always have the option of doing your own thing. Basically, any time the boat is docked (most of the day), you can get off and walk, check out a bike, take a cab, grab a train and go explore the local area. Just be back on the ship before it leaves the port, which is usually late at night, although sometimes it sails during the day. So we had maybe 1/3 of guests doing Excursion A, 1/3 doing Excursion B, and the rest either staying on the boat or exploring the city on their own. So you can do whatever works for your family.