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.
Not all of us put in the time and effort when buying a gift, where those who actually do, are the best of the lot. It shows you care and love this person enough to put that much energy behind something that sounds simple, yet isn't.