Fort Delaware, originally built to protect the cities of Wilmington and Philadelphia, is a Union fortress that once held Confederate prisoners of war. The fort, which dates to 1859, sits on Pea Patch Island in the Delaware River and is only accessible by ferry via Forts Ferry Crossing which runs from Delaware City, DE. Tickets available on the day of visit at the park ticket office (first-come, first-served).
The park offers a number of activities, from exploring the fort to birding (it is the summer home to nine species of herons) to hiking, (the Prison Camp Trail, is an easy 0.8 mile loop over grass and packed earth) to Living History events. 
Visitors are free to walk throughout and around the fort and see numerous artifacts as well as reproductions of items that would have been present in the fort which appears as if it were stopped in time over 150 years ago.
The daily schedule of events has costumed re-enactors explaining life in 1864 with enough to see and do to easily keep you busy for much of a day. Visitors are welcome to ask questions and 
sometimes even to help with tasks.
Visitors learn how everyday tasks such as cooking and laundry were accomplished and witness soldiers preparing to defend the fort as they complete the steps involved in loading and firing a cannon. In the barracks, a soldier welcomes visitors to talk about conditions in the barracks (and maybe share a secret about the ghosts that linger there). Out on the lawn, recruits are schooled in practicing drill as enlisted soldiers.
Included in the ferry fee is the option to travel to Fort Mott, which is on the New Jersey side of the Delaware River. The fort was built up in the late 1800s in preparation for the Spanish American War, as part of a three-fort defense system, along with Fort Delaware and Fort DuPont in Delaware City. This 124 acre state park also offers picnicking, an easy walking trail and special educational events.
There are no food vendors on the island, but packaged snacks are available in the gift shops. Picnic tables and grills are available if you choose to bring food with you.
This was previously published as Step Back in Time to Historic Fort Delaware