Pictured on every street sign (if I'm right) are the arms of William Pepperrell. (The illustration on the left is from the American Heraldry Society Roll of Early American Arms.) The Municipal website of Saco does a fine job telling the tale of the city's formation, but in short it used to be called Pepperrellborough in honor of the town's benefactor. The arms are blazoned: Argent a chevron Gules between three pine-apples Vert.
Unfortunately on the municipal website they've posted an image of the arms as produced by a bucket-shop which is erroneously emblazoned with the tropical fruit pineapple. At the time these arms were likely created, "pine-apples" were the English name for what we now call pine cones. I sincerely hope that when I visit Saco again, that all the street signs with Pepperrell's coat of arms depicts them as pine cones instead of pineapples.
Today, the City of Saco's official seal (right) incorporates heraldry. If you pretend the bordure is a decorative border and that the gradient background is in fact Argent, it's actually quite a nice derivative. My guess or recommendation would be: Argent a chevron cottised Azure between three pine-apples Vert. I suppose they could be pine trees, which would also be appropriate.
Congratulations, Saco on your 250 years and your fine heraldic heritage.