Lots of plants are supposed to have healing properties etc - would she like something like that, or maybe a water feature for bringing elements into the garden. I love it when people buy my plants for my garden, they are an indulgence i might not necessarily buy myself (though i do frequently!) and a permanent reminder of the giver esp if they flowr every year for her birthday. But then she might hate garden!
Obvious ones would be jewellery containing semi precious stones - they all have different properties as i am sure you are aware