D.D.
Does it have an uneven amount on it?
If it did, I might ask the spa if they can split it into 2 cards. So, if it had $136, I might ask if they can make a card of an even $100 (and give that to her) and then see if there is something I would like with the odd amount (eg, put $36 towards a basic mani for me). Or if there is a produce they sell that you would like for the odd amount, like a lotion, use the odd amount towards that.
If that's not feasible - I would have jumped with job for a spa gift card for my 40th :) For any amount!