Bookmarks
Idea sent by Nuala. Difficulty: Easy.
If your partner likes to read, you can surprise them with a personalized bookmark. Photos of both of you, your photos, a nice dedication, cutouts of things they like... With a bit of cardstock and imagination, it can be a curious detail. Any bookstore can laminate it so it doesn't get damaged.
And if it seems like too much work to think about what to put, cut, paste... There are some templates online like these published by Marina, one of our most active forum members:
http://img376.imageshack.us/img376/110/marcador20livro20lovezi6.jpg
http://img175.imageshack.us/img175/7093/marcador20livro20love2pq1.jpg
http://img212.imageshack.us/img212/889/804eba53vv9.png