When we lived in South Brunswick and the kids were little, my wife bought season passes to 'Great Adventure' (as it was known back then...). It was about a 30 minute drive and, as long as you prepacked lunches and did not buy souvenirs, it was economical.
Another cheap trip was going the beach at Sandy Hook. Get there early, no Boardwalk to suck up your money and leave early to beat the exit crowd.
We're now in Hunterdon County. A great local place to camp is Spruce Run State Park. Nice campground, dump station and swimming beach.
Tom and Katharine
'07 Winnebago Tour 40TD, 400hp Cummins
RVing for 19 years & 150,000+ miles