It will be a good stress buster for her, especially if she happens to be a stay-at-home mom, or juggles two jobs. Get her a fully-paid coupon for the entire range of services that the spa provides. Getting a day away from the household chores, or from her job, will be a nice change for her.
This is a good idea for a birthday gift. Hire one of the best professionals to paint a portrait of your family. The surprise factor is not very dominant here, as your mom would also be present at the sitting. Alternatively, you can give the artist a family picture to paint the portrait.
Watching you grow up has been one of the most memorable events of her life, and if you can narrate the entire story in the form of a collage, it will make for a great gift.
At the end of the day, whatever it is that you are gifting your mom, make sure that it comes as a surprise. If she has a clue about what you are going to be gifting her, or if she's aware about it beforehand, the effect will be lost.