A letter telling your parents of all the things you are grateful for. Make breakfast for them, or in the evening, cook a nice meal and set up a table outside with candles. Get them two movie tickets or tickets to a concert or play that they would like to watch.
75! Take a bow, please. Not many are lucky to celebrate their 75th wedding anniversary, so you should be terribly proud of yourselves for being with each other for all those years. The official theme for this year is diamond. I'd urge upon the jewelry again. A ring, a pendant, earrings or simple diamond studs. Awesome, awesome.