Maybe you should collect their pics from their original meet till now. Picking some interesting ones and editor them in a special way~ That will help them fall into a sweet memorry~ Wish your parents happy.

I think the best idea would be almost to try to arrange some way for them to be together on that day or spend more time alone. Perhaps do something like prepare dinner for them with candles and then go out to a movie with your siblings so they would be able to be home alone in the house?