All U.S., Canadian, and U.K. citizens, even infants, need only a valid passport to enter the Netherlands for stays of up to 90 days.
It's a good idea to always carry your passport with you, even if think you don't need one, for example if traveling between the Netherlands and other countries within the European Schengen agreement (which includes Belgium, France, and Germany, but not the United Kingdom). You are required to carry valid ID at all times in the Netherlands, and although it's very unlikely that you'll be asked for it, it's better to be safe than risk a fine.
Before your trip, make two copies of your passport's data page (one for someone at home and another for you to carry separately). Or scan the page and e-mail it to someone at home and/or yourself.