If you're looking for a late-summer family day trip, I highly recommend Hopewell Furnace in Elverson, just over the Chester County line into Berks County and near French Creek State Park. A village based around an iron forge thrived there in the eighteenth and nineteenth century, producing iron for cannons, ammunition and stoves. It's now a national historic site and they do a great job of interpreting the restored village and the iron-making process.
We especially liked the furnace demonstration, even though it was a hot, humid day already. The guides showed us how the stove plates were cast and asked for volunteers to help pack the sand around the molds using heavy wooden pounding devices. As my companion said with amusement, "You were out of your seat before they stopped talking." (It was a great bicep workout!)
The first floor of the ironmaster's house is open and furnished in Victorian style, and the ironmaster's garden is planted with culinary and medicinal plants (I noted that their nasturtiums were doing no better than mine this summer).
In the visitor's center there are interesting displays about the village and a short, professionally produced movie (as a tour guide at a historic house myself, I was beyond envious).
We rolled our eyes at the mandated safety signs warning us that the old stone steps around the site are irregular and not up to modern standards and that the crucible in the furnace is hot. Yes. Thank you.