The best gift you can give anyone is time - do something incredibly memorable with her!
My 35th birthday will be in a few weeks, and life's handed me a lot of lessons. After the birth of my daughter, I was diagnosed with cancer and went through 5 months of chemo. We went to Chicago for a second opinion on treatment and took our daughter (6 months at the time) - Oncology centers aren't used to seeing babies, so she was a hit, and while talking to another patient, I had an epiphany. If my treatment didn't work, and I only had 5 years to live, the only thing I would want is as much time as possible with the people important to me.
Maybe your gift is a girls weekend where you really bond, talk as adults, and enjoy getting to know each other better.
My sophomore year in college, the semester following making Dean's List, I had to stay out of school to earn money. I worked 3 jobs for 15 months, most of which was 7 days/week. Having to pay for a good portion of college was the second best gift my parents gave me - it made me a MUCH better student because I knew how hard I had to work to pay for just 1 credit hour).
She's fortunate that you're able to provide for her, but all the things in the world are meaningless without the people who really make our lives important.
Good luck and congratulations on her graduation.