Not sure if you'd consider this "hiking," but walking along the Delaware Canal is very scenic. There is a nice tow-path, so it's not like hiking up mountains, but it's very nice, and many many miles long.
You can hike or bike anywhere from Morrisville up to Easton. You can basically start at any point and finish at any point. There are old historic buildings and locks in some places, and in other places just woods and wildlife. I took great pictures of herons walking in the water and other scenery.
Here's the Friends of the Delaware Canal website with a map:
Friends of the Delaware Canal - Historical Map