Since no one mentioned riding near Lambertville... Despite it's poor reputation, New Jersey is actually very nice and can be quite scenic. I have done a fall century (Pumpkin Patch Pedal) there a couple times and it was beautiful and the roads are nice. The Fall colors were good too.

Depending on how much time you have you could just ride a loop including some of the flatter part of the state and also the Delaware Water Gap. Or if you have a bit more time ride into upstate New York. Upstate New York's scenery can be absolutely stunning.

The other options already mentioned by others are lovely, but I wouldn't rule out NJ and upstate NY either.