What about taking her to a plant nursery and allowing her to choose a flower or shrub to plant in the special garden that you're going to make.

You could either stop at a cafe or go back to your house for lunch, and then have a private little planting ceremony of the flower or shrub that she chose. I would plant it in a large tub for now to give it time to get a good start while you decide the layout for the garden. That way it will be easier to move when you're ready.

You could even put the tub in the area that you have chosen for the garden, and if you can afford it.... maybe offer to have a plaque printed with her own special message on it to have embedded in the garden in front of the plant. If you don't have the money to do that, maybe suggest that she might like to do that at some later stage.