When my niece turned 16, we took her on a 4-day trip to Colorado to visit family and hike in the mountains. We had a fantastic time, and she loved having "quality time" with her aunts. So, I agree with the other moms that having a unique experience with your daughter is a great idea. Ask her what she would like to do with you for a weekend (or even a day trip). She'll remember it forever.
Another idea is to create a special scrapbook for her. Ask relatives and friends to write her a short letter listing all of her wonderful qualities and why they love her, and add that to the scrapbook. You can also include baby pictures, a letter from you and/or her dad, and any other keepsakes you'd like her to have. It would be something she would have forever.