The 25 year mark is getting to be quite rare in this day and age, so that's really reason enough to celebrate. The official theme for this year is silver. That's more like it, right? Cause there's sooo much you can do with this theme. Apart from the obvious choice of jewelry, the other gifts that you can present include silverware, engraved silver key chains or a ƈ and 5' days vacation at an exotic location (second honeymoon, why not).
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.