Just a short distance (5 min. drive) from center city Philadelphia is a “forgotten” Penn Treaty Park.
In the fall you might be able to see a River City Festival in the Park.
It’s located in a vibrant neighborhood called Fishtown.
To learn more and see lots of great photographs of not only Philadelphia but also Fishtown take a look at: http://www.fishtownonline.com

valley forge is beautiful this time of the year.

Check out the PA fall guide here, too: http://gorp.away.com/gorp/features/fall/fall.htm.

While there are loads of places to take in the fall colors, I would recommend heading north to the Delaware Water Gap National Recreation Area (http://vacation.away.com/pennsylvania/travel-v2-cid358223-delaware-water-gap-national-recreation-area-cid350266-active-and-adventure-vacations.html) or south for a scenic drive through Pennsylvania's Amish and Dutch Country. A little further afield, though arguably the most spectacular region for fall colors, is Allegheny National Forest (http://away.com/tripideas/allegheny-national-forest-backpacking-301092.html), toward Pittsburgh and Buffalo, New York. Find lots more useful information, recommendations, and up-to-date foliage reports at: http://www.fallinpa.com.