There is nothing within the state that, as far as the US Congress is concerned, is worthy of being so designated.
It is customary for senators of a specific state to have "veto power" over any such designation (ie, unless both senators from a state agree, the US Congress will invariably not attempt to designate an area for inclusion), so it is POSSIBLE that no senator from Delaware has ever wanted a NPS site.
There are two very nice national wildlife refuges within Delaware, both worth a visit if you like seeing animals in truly wild settings.
Delaware
Delaware.
Delaware
Though it has several state parks, Delaware is the only state with no national park units. It does, however, have a park trail passing through it (in addition to Virginia, Maryland, and DC).
The state that has the fewest national parks is the state of Delaware. The state is the only one in the US with no national park.
There is a National Park in every state except Delaware. For a full directory, visit www.nps.gov.
Delaware State Park was created in 1952.
The state of Delaware just recently gained their first national monument. In March of 2013, President Barack Obama created the First State National Monument, which is a National Park Service unit that lies in Delaware and also Pennsylvania.
Fort Delaware State Park was created in 1951.
Bellevue State Park - Delaware - was created in 1976.
Delaware Seashore State Park was created in 1965.
The address of the Fort Delaware State Park is: P.O. Box 170, Delaware City, DE 19706