You wont' find it in any travel guide, but there is an amazing roadside attraction at Bob's Sub & Cone in Wellfleet, Cape Cod: The Crankiest Kid On Earth! His name is Dan and he works the ice cream window. I can't guarantee he'll be there if you go, but he's in a league of his own when it comes to preteen attitude. And the ice cream his mom makes him sell is really excellent too! Make sure you stop in for a scoop or two before or after heading to Marconi Station and Beach.
There are two parts to the area: the beach is picturesque, nice for relaxing and reading, not as ideal for swimming as it is ice cold (even in August), rocky and leaves brown booger-like seaweed all over you. The other part has free parking, some absolutely stunning dune pavilions for spectacular views, and an hour long trail called the Atlantic White Cedar Swamp Trail.
This hike was a highlight of my recent New England tour. It takes you from the sandy dunes to a pine forest, then into a huge swamp with a series of wooden bridges to walk on (and enough bugs that you can hear them constantly in the air). This is a great afternoon stop off or destination if you're visiting Cape Cod.