Hi A.,
I like the suggestions of cooking because it is a very active thing to do and has a nearly instant payoff of great food.
I recommend doing something crafty like painting, sewing, crochet, knitting, beading jewelry, belly dance or hula classes (good exercise and way of exploring her blooming sensuality), hiking/biking/tennis/swimming, explore her spiritual needs and go about satisfying some of them, throw potluck party with your girlfriends and invite your stepdaughter, tell everyone else to help you look cool.
Say grace together over every dinner or breakfast, sculpt or paint together, play chess or write a story together. Find examples of prominent mother-daughter teams. Heck, even the Bush women wrote a book together. Hilary and Chelse campaigned together.
Anything physical will help reduce strain and stress.
If she has a strong interest in hair, makeup, perfume,or jewelry; then consider starting an at home business together like selling Avon or Mary Kay cosmetics. Do a pie-baking business during the holidays. Do something to combine or develop your joint interests or indulge in one of hers. Learn a musical instrument and become a duet or create a band.
Start a movie or crafters club TOGETHER. Teaching her stuff she wants to learn will help to turn her around. You and your girlfriends can start a Mother-Daughter Club doing fun stuff with your daughters, and teach them useful and important stuff about life.
In Amazon sisterhood,
B.
P.S. Whatever you do, don't wait; start immediately.