The New Jersey Waterfront Walkway from Hoboken to Liberty State Park is a great ride, although it is not a rail trail. The Henry Hudson is also a nice bikeway, but I can't provide much info about it.
I've included a snippet from the New Jersey page of my website describing the Edgar Felix Bikeway below. Please note that my most recent ride on this bikeway was in 2003.
I found this one day while on business on the Jersey Shore. It was a pleasant surprise, with a refreshing beachfront finale. As of my last ride in July 2003, it was still a work in progress, ending just short of Allaire State Park. A more recent "virtual" visit to this trail via Google reveals that it has now been completed all the way to the Allaire park entrance.
That being the case, load up your bikes and set your GPS to Allaire State Park in Farmingdale, New Jersey. This park's website describes it as ".. probably best known for its historic 19th-century iron making town, Allaire Village, and its antique steam trains on the Pine Creek Railroad. The Manasquan River, which winds through the park, attracts canoeists and fishermen. The river’s floodplain provides habitat for over 200 species of wildflowers, trees and plants as well as habitat for birds and other wildlife. Hikers, mountain bikers and horseback riders enjoy the many trails in the park." From the park, the bikeway heads east for roughly 5 1/2 miles to the beach town of Manasquan.
About a mile and a half from the trailhead, be sure to stop in and visit the Allenwood General Store, right at trailside. Owned by John and June Herbert for more than forty years, this is much more than just a place to get a drink or a snack. It's an old time general store that is filled to the brim with Americana collectibles and antiques, all for sale. If you're lucky, you may get a personal tour of the store's several rooms by the owners themselves. Don't miss it. And be sure to look for Santa's sleigh!
The trailway itself is an absolutely flat former rail bed (see the old RR Crossing sign just across from the Allenwood store) that passes mostly through residential neighborhoods and open fields. I rode it last on a Thursday afternoon, so it was quite empty. But, my guess is that this bikeway is packed on weekends and summer evenings. And why not, when the payoff is a dunk in the beach, a refreshing gust of cool ocean air, or drinks by the ocean side at the end of the ride.
From the Manasquan end of the bikeway, you can make a right and get on Main Street, which will take you all the way to the beach. However, if you're not up for riding in what might be heavy traffic, you might be better off turning left (stop at Dairy Queen for snacks and cold drinks?) and making a right turn onto Atlantic Avenue, a slightly less busy county road with narrow shoulders. Stay on Atlantic for about 1/4 mile until you reach Broad Street. Turn right on Broad Street and make a quick left onto Woodland Avenue. At Parker Avenue, make a left and follow it to the right, where it becomes Stockton Lake Blvd. Stockton Lake Blvd eventually curves to the right and becomes North Potter Avenue. Make your first left off of North Potter onto Ocean Avenue and continue all the way to the oceanfront, where you can lock your bike and do whatever you please. Pizza, hot dogs, sandwiches, and drinks are available right on the boardwalk, and admission to the beach is $5 (July 2003). There are also a couple of outdoor cafes and a night club a block from the waterfront. These are worth the seven-mile ride alone.