I have always loved lockets—the idea that you've got a secret is very appealing. They're great for weddings, whether a photo of your sweetie around your neck or a photo of a lost loved one pinned to your bouquet. A locket would also be a nice gift for a bat mitzvah, sweet 16, or quincenera, because what girl doesn't love jewelry? :) You could pre-fill the locket with a special photo or leave it up to the secrets of her heart.
Some great lockets of all shapes and sizes:
MStevenson Designs (AWESOME)
Old But New