S.M.
My condolences to you.
I don't have a really specific idea but... I am very sentimental, but I hate clutter. So, one thing I do is put trinkets away for the Christmas tree. We hang old baby shoes as ornaments, pet tags from our beloved pets, things we have collected from vacations, etc. That way at Christmas, we have some special memories with us and not just the same ornaments as everyone else. Maybe you could make some thing like that - like a photo ornament or sachet.
I also am a big fan of plants, although it can be touchy if they die. Maybe you could make a donation to plan something or purchase a bench or something at a park or botanical gardens that all could visit.
When my grandmother died, I made matching books for me and her other grandaughter (my cousin) with some recipes, a few photos, and a couple of stories. I typed them and put them in a scrapbook with a pretty cover.
You could also make a quilt or quilted wallhanging if you can identify some of her favorite clothes, pillows, etc. Assuming doing so wouldn't upset anyone.
Just some ideas.