Make sure to stop by Independence NHP while in the city and see all the amazing places and artifacts that Cecelia gets to work with. Website link will direct you to the Independence Hall ticket website. Tickets are $1. All other sites in the park, including the Liberty Bell are free to visitors.
On your way from Philadelphia out to Downingtown you will pass Valley Forge National Historical Park, where you can drive around and see a famous site from the Revolutionary War and the House that Washington stayed in during the winter of 1777/78. This park is free to the public and also has some fun walks and hikes along with the historical significance.
Longwood Gardens is a botanical garden that consists of over 1,077 acres of gardens, woodlands, and meadows. $32 for adults, $28 Seniors (65+), and $17 kids (5-18).
The largest mall on the East coast, makes for a perfect stop to pick up any last-minute items that you may need or just for a fun shopping trip.