maybe you can take the day off, prepare the house for something nice ... Arrange the dining table, buy her a birthday cake, but her flowers, cook lunch or dinner, don't bring all stuff together and let her find those one by one in different places.

If there is anything that she wished to have before and you know it, just buy her that also as a gift and let the gift be the last thing to give her, surprise her with that while she surprising her with the effort that you took to arrange all of this would make her forget that there might be a gift in the end ...

Sorry i guess I gave you a plan, and not a gift idea ...
Good Luck ...

Do not dwell in the past, do not dream of the future, concentrate the mind on the present moment. (Gautama Buddha)