You also have Lebanon State Forest down there. There is a section called "Mt. Misery" which I believe is/has been some sort of campground for religious retreats. There are miles and miles of dirt roads and hare scramble trails there. Lots of deep sand in the summer, but it can be fun in the winter when everything is frozen.
If you're a real old-timer on the NJ mtb scene, you may remember the mtb races put on there in the early 90's. Some of the trails contain seemingly endless strings of whoops which will take your lower back prisoner.
There is lots of ATV riding in there, as well as the really cool sight of the mushers who use the dirt roads to work out/race their dog teams with wheeled carts in place of sleds.