A.L.
A charm bracelet whereby you could add a charm for each special occasion.. Nowadays, they have some really cute ones. So by the time she is 18, she would have charms for each milestones/special event. You could start the bracelet out with her first charm, a charm from you that would be meaningful. For example, if she plays an instrument, then buy one that is suitable...
Also, money is usually big with teens.