What type of spa experience are you thinking of doing? Massage? Facial? Foot rub/pedicure?
I've only ever been twice. Once for a facial and once for a massage...but I'll tell you my favorite parts:
~the hand massage: oil or lotion the hands. Wrap them in a hot towel for a minute or two. Massage one hand at a time, gently sqeezing and stretching fingers, and using both hands to gently squeeze palms and rub backs of hands.
~foot/lower leg massage: lotion or oil legs and feet and rub down legs and massage feet. Of course, if you had her feet soaking in a nice epsom salt+essential oil bath while you did her hands, this would make it all the more wonderful!
~hot rice pack on back: this feels so good. They used a large sized rice pack, but you could sub a hot water bottle or two wrapped in a towel. Or maybe make your own rice pack.
~facial: you can keep it pretty simple. Pull hair back in a stretchy band to keep it out of her face. Put a light oil on the face (coconut?), and then wrap with a steamy (but not too hot) towel...leave a little air hole. Wash down and rub face with a damp warm washcloth, and apply a gently toner and a moisturizer.
Find out her favorite smells and/or flavors. Vanilla is always gentle, and can tone down other scents (like orange+vanilla). Coconut oil is a great moisturizer, and can be blended with essential oils for some scent. If she's sensative to smells, then keep the scents very light, and maybe just use one or a blend of two gentle ones for your products.
Beeswax candles are nice. Dim lights, soft piano/harp or flute type music are wonderful.
If you want to make your own products you can use:
~extra virgin coconut oil for the moisturizer/massage oils, with your own essential oil mix blended in.
~witchhazel for the toner with a few drops of essential oils shaken in
~epsom salts with essential oils...I love peppermint or eucalyptus blended in for foot soaks and oils.
~I would also use Dr. Bronners chamomile soap (or unscented) to clean off the oils and wash her face. It's very gentle.