A great gift would be like Karenne suggested, just a small album. A small album is just the right thing, especially if you are not sure it is something she will carry on. These albums hold about one picture per page and it would be really fun if you get some pictures of her and him before they meet and put them in the album. Have some engagement pictures and some journaling there and it is a great heirloom. What a great way to start of their married life with before and after pictures already together. Its a great way to have something to show their future kids.
Let me know if you have any other questions.