Don't give an item. Spa treatments, weekends away, dinners out, concerts, etc. tend to go over better.
If I were you, I'd take her over to a resort sort of place on the coast, get her a spa treatment/massage/facial/etc., take her out to eat, and put in some quality time with her.
Spend some time planning it so everything goes smoothly, is taken care of and there's nothing for her to worry about. That will be greatly appreciated. Don't tell her where you're going, either. Just give her a note telling her to pack for a day or two away with suggestions for outdoors/formal/whatever clothes to bring.