You tell us nothing about your aunt, age , fitness, what she did before retirement, her likes and dislikes. Any new activity needs to be something that addresses her existing interests and inclinations, not just an idea grabbed from the blue. She also needs to want to find an abiding interest.
If she is a people person, some kind of voluntary work may interest her. Most counties/towns have a Volunteers Centre. If not most council websites will have a volunteering section. The variety of opportunities goes well beyond Meals on Wheels and working in a charity shop. I became involved in the Churches Conservation Trust through seeing an advert on the Volunteers register on my county council website. I went to the website initially for something entirely different, road repairs or something, but wandered round it and found this site and opportunity.
Does she have something that really interests her, say, never misses a wildlife programme on television, in which case, her local Wildlife Trust might be interesting, they always need volunteers to do everything from office work, wardening, to guiding visits, ditto National Trust.
I think you need to think constructively, starting with who she is, what interests her and what she would be willing to do and, almost definitely, someone to go with her to the first contact meeting, going alone can be daunting.