If more than a brief shower is expected, bring a hat with a bill (I like a baseball cap that I can attach to my belt). With the hat on under the poncho, rain doesn't get in your face.
Also, if it's a warm day and the rain isn't a thunderstorm, head for a water park. As long as there is no thunder/lightening the rides will remain open and you'll have the place practically to yourself. I believe the saying goes "you can't get any wetter than wet".