You can buy her flowers, a perfume, chocolates (its fine o pamper her with chocolates sometimes) a book she might enjoy reading and put it all in a beautiful basket.
A birthday newspaper is another interesting option. It is nothing but a customized newspaper of the year your grandma was born in. She will enjoy reading the events that happened on the day she was born. You can get it framed for her to put it on the wall of her house.
You can buy her an elegant pair of earrings or a jewelery box for her to keep her small trinkets that she keeps forgetting here and there in.
If she keeps forgetting her spectacles here and there and then spends half a day looking for them then you can also buy her a sleek chain for her spectacles. It can be attached to the specs and when they are not on her eyes thsy can keep hanging in front of her. She will really appreciate this help.
Hope the list helps you decide what you plan to gift your grandmother on her birthday. Something unique, sweet and thoughtful will definitely put a smile on her face.