Oooh, oooh, oooh! *my Horshack moment*
My boxing gym is located within a community center. At this community center there are a variety of activities, for any age group. AND, it's relatively inexpensive! Does your mom live by a community center? If so, does it have a online site where you can check out what classes/activities it offers?
There are such things as swimming, wellness courses, yoga, etc. I know you said your mom may not be into yoga, but it's worth a shot to see what else they got. It'd also be good for socializing and meeting up with others within your mom's age group.
Or..as THeMaCHinE suggested: Richard Simmons.