Re-live the Civil War at Fort Delaware State Park
Fort Delaware is a Civil War-era fort and one of the main attractions of Fort Delaware State Park on Pea Patch Island. To get to the park, you need to take a ferry from Delaware City of Fort Mott State Park (also a must-see if you're in the area) on the New Jersey side of the Delaware River.
The fort itself is in surprisingly good shape and is located actually out in the water a few feet off the island. They've resorted some of more interesting areas of the fort like the barracks and the mess hall and now offer guided tours to visitors. One of the coolest parts is seeing the massive cannons on the second floor and see a live demonstration of a Columbia black powder gun from the era. One of the nice detail is that many of the park employees dress, speak, and act as the soldiers did in the 19th century so it really helps bring the place to life. You can see an old-fashioned blacksmith's shop, for example, or help the costumed wash-woman work a washboard and ringer.