Consider some animal therapy work. I have known people that had severe social anxiety open up working with animals.

Why not also do therapy/social skills group in conjunction with other activities. She can learn some strategies and then practice them in real life. My DS is much younger but these seems to work for us.

How about working on the crew of a play. And eventually trying out for a part in a play. I was shy as a kid and would never have wanted to be on stage. One of my dc is like this but has been participating in plays every summer. It is a way to gradually overcome fear of others noticing you. The more you avoid your fears the worse they can become. With a play everyone works together some with big speaking parts and other kids doing important behind the curtain work. Or how about learning to play an instrument or taking a writing class.

Membership for say a month in a gym if that is possible or community center where she could choose from a wide variety of classes. Some might relate to stress reduction without saying so such as Yoga or swimming.

Good suggestions, too, on possibly an arts center

Volunteering with a group which would only have lots of "hugs and smiles" such as Special Olympics in your area as this program involves a wide range of volunteers. Also, along the same lines, I might suggest seeing if the therapeutic rec program in your area could use an extra person for an event, a class or an outing. In supporting others, she may well release some of her inner anxiety.

You are doing the right thing not "to push" too much as it is obvious that she has made some decisions which work for her. There is much to be said for how life changes when the pressure cooker of high school years are behind one. While it is hard to see her struggle so, I would say she has a whole life ahead of her.

Our oldest has a kind of inner driven anxiety, but she also did complete college (with some bumps in the road), has a great government job, married and has twin five year olds (difficult for her at times, but she says well worth it). The most interesting aspect was that when in Latin America on various internships and volunteer work, she had no issues with stress/comparisons - why because she basically knew no one to compare herself to.
Now with the appropriate therapist and psychologist she does maintain a good life balance. And key she has the support system in place when there is a blip. You are doing the right thing by your daughter, and I know it is so hard to see one struggle so.

A chess club.

Teen Meditation Groups:
http://imcw.org/Community/ChildrenandFamilies.aspx