Welcome to the board. What a sweet grand daughter... and great granddaughter you are to reach out this way for others.
If Great-gran is limited to what she can do from her chair, that narrows her options a bit. Folding clothes and towels, perhaps. If Grandma grows veggies in her garden, Great-gran can help snap beans, shell peas, etc. Some other food prep chores may be appropriate, but it would depend on her ability to safely use a peeler or knife.
She might enjoy running junk mail through a shredder... hmmm. Can she do her own mail sorting and bookkeeping?
Also, anything she does for her own care and grooming is a help, although she might not realize it.
I had an uncle who was quite deaf. He always said the mealtime blessing because even if someone else did it, he couldn't hear it and thought it hadn't been done.
Well that's what comes to mind right now.
Please keep posting, and let us know how it goes.