Japan trip in December / less known places?

Hi everyone,

I am in the process of planning a two weeks trip in Japan for December.

I have been to Japan for three weeks with my partner last September and did the classic Tokyo-Kyoto/Nara-Osaka (plus Hiroshima and Miyajima day trip) itinerary, and then spent some time in Kamikochi, Matsumoto and Nagano.

This time around I will be bringing around my mom who’s in her 60s and can still walk a decent amount, but not as much as us so we will need a slower pace.
We will be spending probably half the trip in Tokyo since there’s so much stuff to do. As for the rest, I am looking for recommendations on other places that are a bit less known. We loved the part of our September trip in smaller or rural places and I’d like to show my mom this kind of vibe.

I’ve been thinking of using Sendai as a base but I don’t know whether we will be kept busy for a full week there, especially since we won’t be able to do hikes or long walks.
I am not necessarily looking for places specifically in the Tohoku region, but places that aren’t too far away from Tokyo – for example I’d love to see Hokkaido but I am not inflicting that long of a train ride on my mom.

Do you guys have any recommendations?
Many thanks in advance!

Edit: I forgot to mention that we are not planning to rent a car.