The gift of her grandmother

Give her your time. Maybe a birthday card. And some candy. And some warm socks. And some house slippers.

I think the suggestion of a gift of time is a great one.

If that's not possible (as in, she's an ocean away from you), then perhaps something which is either more like an experience or something for her convenience, health and safety. Much older folks are often trying to get rid of stuff, so a knickknack isn't a good idea. So theater tickets could work (or even tickets to a game if she likes sports, that sort of thing). Convenience, health and safety kind of stuff is house slippers with good treads on them, or maybe a basket where her remotes can go so they don't end up in the couch cushions. Or a subscription to a good meal service which can deliver wholesome food so she doesn't have to cook or shop.