I went on a jr. class trip to somwhere in N. Jersey... I want to say Great Gorge. There was ice skating, skiing, and other stuff.
For spring time, there were alpine slides, and other stuff. There are quite a few resorts of this nature in Northern New Jersey, do a search and call it N. Jersey resorts... or skiing resorts.
You could do Atsion Lake/camping up in Wharton State Forest. (near the Southampton/ Tabernacle border) The lake may be open earlier, and you could wind up doing a nature/Pinelands lesson as well.
Clementon Lake Amusement Park is nice as well, smaller but still works.
Great Adventure doesn't open until April 5th.
There's a cool area off of Rt. 72... its a old terra cotta factory which has now "run down"... it was around back hundreds of years ago. You can see the ruins, and figure out why the factory closed.. you would never know its there, because you just basically pass it on the way to LBI.
You could do a lesson on the "lost" towns of N. Jersey, Ong's Hat, this town, and other things.
Island Beach State Park. Yes, its a beach, but the nature trails are fabulous as well, and its really great.