Gift Card
I would go with a gift card, since you don't know the fiancee and probably not their tastes. Something for a nice dinner, a home improvement store (if they're buying a house), or the movies.
If you would like something more personal, you could always get an engraved picture frame with their last name on it. I saw some that weren't very expensive at the mall, and they looked really pretty. They will need picture frames after all of that photography at the wedding

I wish I had gotten a nice 8x10 frame.