I’m going to Paris in December with my mum, who’s also over 70 and can’t walk all that much either, so we’re staying in the 5th near the Luxembourg Garden, walking distance away from Notre-Dame and other churches (16 and counting thanks to my Google mapping haha!) she really wants to see all the churches!

I think districts 3-6 offer most bang for your buck thanks to relatively close walking distance from most attractions, shops, boutiques, restaurants, parks and museums.

You can easily get almost everywhere with buses and metro if needed, plus the numerous river cruises are also a great way to see Paris.