It's really hard to answer this, you said she's an acquaintance and that this is a token of appreciation for something in the past, also you have to consider her current romantic circumstances. For example you can't get another mans wife a 'chocolate body polish & 60 min. hot stone massage' at a spa without the husband thinking that's a little odd but you can get his wife a gift certificate for a manicure at the little spa down the hall from her office. One will seem like a come on, the other will appear to respect her boundaries even if your intentions are not.
Now as far as what's a nice gift, take your cues from her. If she has nicely manicured hands then she'll appreciate a manicure and/or a pedicure (women who manicure usually pedicure as well).
Massages are always popular and the 'upgrade' of scent, hotstone, etc. is a nice and thoughtful touch if you have romantic intentions, if you do not then keep it to something theraputic, where you can say "I thought it'd be nice for you to have an opportunity to relax, destress."
Specialty services like body polishes, facials, etc. should be something for someone you know romantically or are involved with. They are fun, great gifts but a bit personal. Partially because they are tailored to her lifestyle but also because if she wants to come back and tell you about it later you are going to get personal. If she says thanks for the massage she will just say, the masseuse was great, very relaxing, etc. If she tells you about a body polish she will have to talk about the sugaring, or the chocolate slather, mud, etc. She'll be naked on a table with another woman rubbing some sort of cream, food product or abrasive material on her - it's great - but it also might be far to personal for a woman that is just an acquaintance to tell you about.
As far as a facial, well you'd need to know about her skin type etc. to get the right one and asking any woman do you have dry, oily, etc. skin. Do you worry about wrinkles, fine lines, etc. not going to go over too well.
As far as getting a gift certificate for a $$ amount vs. a service. My thoughts are pick a service, again just a basic one and stick to it. It's personal and says I wanted to get you something nice, I tried and I'm doing something nice. She'll appreciate it. Cash amounts are okay but I have always preferred when someone picks a service instead. It can open a dialouge as well, if we are involved romantically I will tell them "That's great, I really like the massage, perhaps sometime we can get the couples body polish and massage package" or I will say "Thanks, I usually go there for my manicures and have never tried the massage." This way you will know what I do and do not usually do. The cash amounts usually just entice me to spend more of my own money (as the amounts tend not to be exact to the services I'm using) or the money goes unused which is a waste. Oh and to be clear, it's not that I mind spending my own money but I'm pretty sure that's not the intention behind giving someone a gift.
I hope that covered all the bases.
