More than like that she doesn't want or need any gifts, almost all people I know over the age of 65 has told me that. They're usually in a state of getting rid of things rather than acquiring more things.
But that doesn't mean you want Christmas to come and go and you are empty-handed.
Keeping in mind this is usually just something that makes you the comma the giver feel better rather than the receiver (but, it depends on the person... Some people really love to receive gifts. Most of the seniors I know find it to be uncomfortable. They have to put on a happy face to receive things that they know they're not going to use / want but, they usually can regift, so not typically a big problem)
My recommendation is to go with consumable items. Nice things that you think she will like, that are not so incredibly perishable that they need to be used within a few days. Homemade treats are usually well received, signifying more effort / love on your part as the giver, however store-bought nice little lovely treats are also good because they can be more easily regifted/given to other people. (Some people do not like homemade treats if they do not know who it's from.)
You can always try to give items that would give comfort such as a warm throw or slippers, keeping in mind that she probably has enough throws, and slippers can be a bit of a personal item. Does she prefer to be soft does she prefer to be all around the foot, does she prefer it to have a hard soul, does she prefer to be a slip-in type or does she have to be able to bend over with her fingers to hook the back part of the slipper over her heel which might be a problem if she has any type of mobility issue... Plus some slippers are actually quite slippery underfoot or if they're oversized and you think this would be cozy and warm, they might actually pose a tripping hazard... Lots to consider for a 90-year-old, especially if they are not specifically telling you they're likes and dislikes...
If you know that she has a very favorite particular hair or nail salon that she really likes going to you could consider getting a gift certificate for that place for her, same thing if there is a very specific cafe or restaurant that she is able to get to (not just one that she used to go to or likes but actually is able to get to again, mobility issues or travel concerns being a practical deterrent to going out places) That can also be a nice gift but a lot of seniors don't love gift cards because they can't SEE the value that's on the card (if it's electronic) or they are concerned that it will expire before they have the opportunity to use it.