The George Washington Bridge is the only way to actually ride into Manhattan from the west. If you don't mind taking a ferry onto the island I can give you a route that we ride as a club event every year. It starts in Lambertville, NJ. You could follow the AC Atlantic Coast Route south from Delaware Water Gap (which is actually noth of Portland) to L'Ville and then pick up my route. (Virtually all flat as it follows the river south.) It ends in Hoboken, NJ (birthplace of baseball and Sinatra) where you can catch a ferry to Manhattan.http://www.nywaterway.com/Home/RidetheFerry/SchedulesRoutes/InteractiveFerryMap/tabid/132/Default.aspx
Take the ferry from the 14th Street dock in Hoboken to Midtown (W. 39th St.) and jump right on the bike path that runs north and south along the Hudson. During the week you could also catch a ferry or PATH train from the NJT Station in Hoboken to the World Financial Center if you want to visit Ground Zero before riding noth along the Hudson River path.
You can do it from L'Ville in one long day. We start across the river in New Hope, PA and end in Brooklyn Heights. Total is about 96 miles.
If you don't want to ride south to L'Ville just to head back noth, you could cross the river into NJ at Belividere and devise your own route to meet up with it. Be warned though that some of the hills in that area of NJ are quite difficult. Some call them the Jersey Alps.
Most of the route is, believe it or not, quite nice and not urban, and there are several historical sights along the way, although you will need to feel comfortable riding in urban settings and in traiifc. In the 14 or so years this ride has been run, no one has been assaulted or injured by a motorist. You are more likely to get cheered on by the locals in Jersey City. One huge disclaimer: You would be insane to try it during a weekday. That is because the route goes through the heart of the Port of Elizabeth & New Jersey. I go there for work sometimes and I don't feel safe in my car. All trucks trying to make deadlines all the time. We hold our event on the Sunday before Labor Day when the port is deserted. I think it's pretty empty on summer weekends.
Send me a private message if you want the cue sheet.
Another source you might try is the BTCNJ:http://www.btcnj.com/
or the Central Jersey Bike Club:http://www.cjbc.org/index.html
or the Western Jersey Wheelmen:http://www.bikewjw.org/