I like the jewlery idea too, if you know her well enough. Otherwise maybe you could volunteer to watch her kids (if she has any) at some point, or give her a gift card to a restaurant or amazon.com or something. I had sort of the same question, because my doula is in training and won't let me pay her... but I think I'm going to give her a thank you card with cash anyway (via her husband so that she can't refuse it).
Here's the link to my post in Birth Professionals:
Gift ideas for unpaid doula?
HTH!