Giving a gift card to a spa doesn't mean obligate you to go with her. I've given my neighbor who babysits our kids many gift cards to the salon we both like to go to, and it never occurred to me to go with her...just figured she'd like to go alone or take her daughter or mother with her, which is what usually happens, and I'm not offended at all! :)
Although I do understand your concern.
You can also try getting her a gift card to her favorite restaurant or a favorite specialty shop (Like a gourmet shop like Frazier's on 96th street?).
Good luck!