On formal night, have their pictures taken several times, pick the best of the best.
During the captains party , see if the CD will have a special dance in their honor.
Champange, chilled in their stateroom.
Order room service for them late one night, set up a picnic on the bed.
Messages in the spa.
Special sing-a -long of their favorite songs in the piano bar.
Champange sunset cruise (shore tour) ??
Celebrate their anniversary at dinner. Talk to your head waiter.
Some of these suggestions cost $$ and some are free.
Hope your and your parents have a wonderful time with great memories.