Think about the things she has admired when she's been with you. She'd probably really appreciate the fact that you noticed her desire for something. However, since you are both 18, I'll assume you don't have a lot of money to spend on gifts for each other, so I would say give her some personalized gift certificates, with you as the gift - you taking her to her favorite restaurant, you taking her for a picnic or a drive in the country, you cooking dinner for her or taking her to an amusement park, etc. - you get the idea. Make a "redeemable coupon book" and surprise her. That will be more remembered than anything you could shop for at this late date.
Happy Christmas...