For my niece's graduation quilt, I planned early and included a scanned image of her graduation program and the Nightengale pledge (she's a nurse) on the label. Obviously, I made the label after the ceremony and just presented her with the top at the time. I waited to quilt it until the label was already applied since this was a large label and I didn't want it gapping away from the backing.
I often include pictures or some other memento on gift quilt labels. If there's a poem that's relevant or unique wedding vows or something like that, I think those are good candidates for the label. I think this is another place where you can let your creativity flow.