First, let me welcome you to this forum. Your question brings me cheer, and your Grandma is a lucky woman.
In pre-COVID days, the best gift was almost always something that created wonderful memories. Taking your Grandma out to a nice dinner would have been a good bet. But those options are largely off the table these days.
Nice flowers are a good bet along with anything else (like great body lotions) that can be enjoyed and used up. Most of us who are seniors probably have more "stuff" than we know what to do with. Our problem is getting rid of it. If you send her flowers, try to have them arrive a day or two early so that they get a prominent position. Other family members may go the same route and you want yours right up front.
Another good option is to do something for your Grandma that she would not normally do for herself. Having her car detailed (washed, waxed, and thoroughly cleaned) is one example.
You have a great day, and I am sure that what ever you get her, your Grandma will love it because it came from you.