Local special ingredients on Cape Cod
When I lived in Boston, and occasionally visited the Cape Cod area, there was more of a regional cuisine than local ingredients. Seafood -- lobster, fish, clams, mussels -- is the most uniquely Cape Cod ingredient I can think of, since it is right on the coast.

This infographic describes some of the foods that are associated with New England, although that is a little too general for your question -- dairy, for example, is much more a Vermont or non-coastal ingredient. However, Cape Cod is close enough that Vermont cheddar will be quite easy to come by!