Pea Patch Island is a small island on the Delaware river in between Delaware City, Delaware and Finns Point, New Jersey. Given it's position at the entrance of Delaware Bay, the island was once a strategic military stronghold and famously served as a prison for Confederate soldiers captures after the Battle of Gettysburg in nearby Pennsylvania.
Today, it's home to the Fort Delaware State Park and a major stopping point for migratory birds. To get there, you need to catch a ferry at Delaware City. The ferry trip doesn't take all that long (the whole trajectory is less than a mile long) and costs $11 for adults and $6 for children. Many people head to Pea Patch island to check out the 19th-century Fort Delaware complex, but the island itself also makes for a nice visit. You can see large flocks of heron (the largest in the US outside of Florida), egrets, and ibis and there are facilities set up for picnics and barbecues. I'd suggest bringing a cooler of food and drinks to have a nice early lunch in the fresh air before heading out to explore the fort and its cannons, do one of the nature trails on the island (there are signs indicating where they begin and end) and checking out the large colonies of birds.
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.
Aside from visiting the fort itself, a day at Fort Delaware is also a great excuse to enjoy the salt air and wildlife on Pea Patch Island. There are picnic areas, hiking trails, and bird-watching areas around so you can spend the entire day.