I would get her a gift card to the Gap, or Old Navy for maternity clothes. With my first, I didn't show until about 4 months, so that would have been a great present. Heck, I would have liked that with my second as well!
Also, depending on how sensitve her sense of smell is, there might be some scents that don't agree with her. You could always get a Bath and Body Works gift card, then she could choose the scents that work for her.
What about a gc for a manicure/pedicure?
Hope that helps!