If you are going to go down the route of a gift voucher for an "experience" I would recommend Red Balloon.
Something they do that is REALLY COOL... They offer speciality pregnancy massages. You see, if you are pregnant you need to have a very experienced professional looking after you if you want a massage because someone who doesn't know what they are doing can really screw things up. I did not know this, until a colleague of mine was pregnant and could not find someone who could get a massage because she was past her first trimester... SAD!
Anyway... Red Balloon have experiences for everyone and they will help you find the perfect experience for you. Even better... they based online so you can shop in your jimjams!!! :D