Did they give you a gift certificate for the spa? I would just put it in an envelope with a little note that says, "I thought you would enjoy this!" and put it on her pillow or somewhere else where she is sure to see it. Then wait to see what she says. If she says she can't do it because of work, you can say that you had hoped she could take the day off. If she insists she can't do it, then tell her not to worry, you will call the spa and ask them to change the date. Usually they give you a time period in which you can use it.
