I personally have purchased things from the Dona line for friends' weddings, and then slip in something that they would want like a small gift from their registry or small gift card. That way they get something that they want and something unique that made you think of them.
She is extreamly girly and I haven't seen a registry. I kinda want to avoid cash cards because I don't feel they are as personal. This is kinda my frist wedding that I have attended (Mine wasn't like this at all is was very small and very different)
It could fit under money, but I think a gift card is a better choice. If they have a house, something like Home Depot will get used a lot and a gift card would be nice. But honestly, a gift card to anywhere you think they might use it would work.