e're not suggesting Arnold's is a secret, sitting as it does right smack beside the busy Mid Cape Highway with a perpetual line out the door. The secret is what makes it different from those countless other Cape Cod clam shacks, where the fried seafood seems more batter than fish. Arnold's goes easy on the batter, and, more importantly, changes the oil four times a day, while the food at those other places tastes like the oil is left over from when Happy Days (the TV series after whose burger shop, yes, Arnold's is regrettably named) was still in its first run. And if you think that makes the clams and oysters taste good, don't miss Arnold's other secret: the homemade onion rings, which come piled so high on the paper plates, they're hard to carry to your picnic table. No, it's not fancy, but Arnold's reels them in. Top off your meal, if there's room, with a stop at the ice-cream window outside, scooping up 30 hormone-free flavors from Richardson’s Dairy in Middleton. And while it isn't likely that you care, given that you're eating at a clam shack, there's also 95 percent fat-free frozen yogurt and Emilee's Italian ice.