Yep, I agree with Sherry. A nice card with a hand written message, saying how much you appreciate her help. That is something that she would keep for years. And if you make things, do embroidery, pottery, paint, draw or whatever…that is also special as it is something that will always remind her of you!
I just know that they are the things that I cherish most. I have an old fashioned card from a student with a lovely poem inside (I think Emily Dickenson) with a really lovely note that I just love. I also have a hand made mug, quite rough and not that slick, but I really love it. And also a have a nicely framed photo of me teaching a student, that a husband took one christmas – with a really sweet thankyou message on the bottom. And a cushion that a student embroidered for me, with a harp and a lion on it (celtic pattern) and she loved celtic things…really sweet of her to do all that work.