in all honesty, cash. They will need it.
If you really do not want to give cash, check their registry. (ask a family member or someone in their bridal party where they are registered)
That way, you know they will like what you got them.
If they do not have a registry, you cannot go wrong going to a store like Things Remembered and getting a nice engraved frame or crystal frame with their photo in it. (or put a love poem in it when you give it to them)