Fort Mott is the most worthwhile day-out near Pennsville. The fort's earth-covered concrete batteries face the Delaware River and are open to walk through — climb the parapets, look out over the channel, and imagine the gunnery officers waiting for an enemy fleet that never came. The interpretive trail along the moat does a good job of explaining how the three-fort system (Fort Mott in New Jersey, Fort DuPont in Delaware, Fort Delaware on Pea Patch Island) was supposed to work together.
The grounds are also a pleasant picnic spot, with views across to Pea Patch Island and a ferry service that runs in summer to Fort Delaware. Allow an hour to walk the fort, longer if you take the ferry. Admission is free and the park is open daily.