The box idea is great. I had a college boyfriend who gave me a decorative box with a rock inside from the beach. I still have both.

And if you're not the sentimental type:

One Valentines Day my ex husband took my car and got it cleaned- inside and out, and detailed. That was the best gift I ever got. With everything else I was juggling, the car was always trashed. I hated driving it that way but didnt have the time to clean it up. It was the perfect gift.