Hi there,
Make her a big hamper. Get an empty basket and fill it with all the stuff you know she likes to eat & drink. Stick in a few baby goodies (like a little photo album or a keepsake box for his/her 1st lock of hair), but try to make it mainly about her. She doesn't want to feel like she's just a baby carrier, so that's why I'm saying focus mainly on goodies for her.
Also, you can get a very nice oil from Boots for pregnant women called Bio Oil which is lovely for a nice little Birthday massage of her bump and legs etc. It's specifically for use in pregnancy.
Oh, also, if she likes excercising, you could always book her a few sessions of pre-natal yoga. You can ring round a few teachers in Yellow Pages and they'll be able to direct you to a class for mums to be.
Good luck!