What about buying a pretty photo album and filling it with pics of her in a sort of This Is Your Life style? Or you could fill it with pics of her loved ones and friends.
Depending on how much you are looking to spend, and booking availability, how about a trip in a limo? We did this for my Dad's 80th and he never stopped talking about it! You might be able to club in with other family/friends to make it a combined gift?
Providing she has a garden, you could always buy her a Dove Cote or slated bird table. She could then sit peacefully & watch the birds coming & going from her windows.
If you have access to any photos of her in her younger years perhaps you could have a photo enlarged & framed...or a photo of something or someone that she cares about.
Thanks everyone who gave me ideas for the 90th Birthday gift. Although she is my cousin I don't know her well so I have bought a nice box of Thorntons Chocolates and found a young photograph of her and have modified it on the computer to attach to the box of chocolates. Thanks for the help.