Gift certificates are nice, but you kow what I have appreciated even more? Money. In a few cases clients, when paying the remainder of their balance, have added onto the amount they owed. It is always a surprise, and it is always appreciated. One client, when I called to thank her for this unexpected bonus said, "Well, we know you love to read, and we thought about a gc to a bookstore; we thought about a spa; and then dh just said, 'Christmas is coming and she has kids -- let's just give her money.'" It is always touching when a client tacks on an extra $50-100 I wasn't expecting.
But there are still good ideas out there -- a gc for a bookstore would be appreciated by almost any doula as there are always books we would like but often we can't justify just buying them for no good reason.
Something she is presonally interested in might be nice -- a membership to LLL which she can add to her list of groups she supports when giving clients information, or a subscription to Midwifery Today to give her additional resources in this area.
A gc to an eating establishment is nice. Another thing I have enjoyed was a gc to a hospital cafeteria, lol -- it was nice to know I could grab a bite to eat and not worry about writing a check (I so rarely have cash), and they don't take cards at this cafeteria.
One thing that I always love receiving is a card or note where mom shares with me her feelings about having me at her birth -- I save them all and reread them often.