The D&R Canal Park is a long and narrow bridle path that stretches about 60 miles. I have put in around Blackwell Mills in Franklin and find that this location offers the best scenery as well as the easiest access and parking.
Put in by Blackwell Mills. Park your car by the gate to unload and then move the car to the parking lot just a few yards away. Don't leave your car at one of the gates or you'll get a nasty ticket!
Paddling the D&R canal offers a wide variety of scenery coupled with flat water. If you put in further north (say by South Bound Brook,) you'll see a bit more industrial ugliness than you'll care for.
Putting in by Blackwell Mills offers a nice scenic paddle that makes for a great entry for friends that haven't paddled before. Up the canal a bit is a canoe/kayak rental place if you don't have your own equipment.
You'll see cranes, ducks, turtles and lots of butterflies as well as lots of greenery. Enjoy!
Porta-potties and plenty of parking.
538 Canal Road, Somerset, NJ
Take route 514 west to route 533 south. Make a left on Blackwell Mills Road. Parking lot will be on your right and right after that is one of the gates to get access to the canal.
Ed Gertlers Garden State Paddling (of course)