Elderly Grandmother doesn't wish to do anything. Any ideas?

I don't much believe in medicating what is an honest response to the losses in life. We cannot make ourselves happy with pills. That said, this is a relatively young woman by our scale today. I am 77 and I cook, sew, garden, walk, visit museums, read, listen to podcasts, watch a bit of tube and am well and ready for bed at that point.
Others are different. I had an Aunt much like your grandmother. Once I asked her if she was sad and she said she was not. Have you asked? It is an honest question. My Aunt said she enjoyed sitting by the window, watching the birds at the feeder at the assisted living, and thinking of her entire life, her husband, the homes they had, the dogs, the trips they took and etc. That is to say she was content.
You Aunt could easily live another decade or two. Time to ask her. Are you "content". Do you miss your husband? Have you any interest in reading or listening to pod casts? In learning how to use a computer? Is there anything I can do to make your life better? You sometimes seem sad to me, but perhaps I am not seeing it right. Speak to them. They can listen and respond. Believe what they say. Go from there.

I agree your grandmother needs to see a doctor about an anti-depressant.

One component of depression is an inability to initiate action. The person knows they should go to church, but cannot get off the couch and get out the door. Often the depressed person will take action in company with others. Can you get her to take a walk around the yard or around the block with you? Ride in the car for a quick drive thru trip? Can you and other family members sit with her on or around the couch and watch a good older movie? (Disney+ has some good old comedies.) Ask her to help with meal prep (my Mom likes to peel and dice fruit and veggies)? Setting the table? Folding laundry? Any little thing you can do to pull her even briefly into the life of the household. Activity and the feeling of being useful or needed can help lift depression.

If there's a senior center or adult day care program available in your area, make every effort to get her there, even if you have to go with her the first few times. The socialization opportunities and activities at the center can make a big difference in your grandmother's quality of life.

Please just remember you are not responsible for your grandmother's happiness. You can help provide her opportunities to find moments of joy, contentment and maybe even happiness, but she must also being willing to reach for them. Also remember you are at a different stage of life than your grandmother. Like you I cannot imagine a life with some stimulation to pass the time; not only can I not imagine having the opportunity to read a book and not taking it, I cannot even imagine _ever_ reading all the books I am interested in reading! But my mother, the woman who created that life long love of books, now spends an hour or two just looking out the window. She tells me she is remembering working the farm with her older brother or visiting her grandmother's house and playing with her cousins. Most of the time the people in those memories are dead. At 87, my mother is in the one percent survivors club. She enjoys remembering and talking about the people who were important in her life decades ago. I remember visiting other older relatives and how animated they would become talking about gatherings or experiences from their younger years too. I think maybe this is just a part of aging.

No, at her age that doesn't seem normal. Maybe she has undiagnosed depression. Or, she has early dementia (70s is young for that nowadays). She should be taken to her doc and these 2 things can be addressed at the same visit.