On our last cruise we were gifted the flowers and state room decor from different people. I actually found the flowers got on the way more often than the decor and really thought about asking to have them removed. We were constantly moving them around the room and never really found a good spot. They were still in perfect shape at the 7 day mark when we got off the ship.
Is she 18 yet? If so, maybe a rain forest pass, a certificate for a mani/pedi, a card for Cove Cafe, or a meal in one of the specialty restaurants. If not, you could send her some on board credit to do some souvenir shopping.
I also like the idea of letting her pick a special excursion that you wouldn’t normally do. It could also be fun to deck out your door with graduation stuff. I’m sure the local party supply would have some garland or something fun. You could also do matching family shirts one day with Disney characters that have “ Mom of the graduate”, “sister of the graduate” and so on. None of that would be there when you arrive, but setup fairly quickly.