There is a very nice zoo in West Orange, maybe a tiny bit out of your way but definitely worth a diversion:
They have lots of play areas, a great carousel, farm animals that you can pet and feed, a train ride,etc. etc. They have had a few major renovations recently and it's a really fun place. They have a decent cafe as well, you can get pizza, grilled chicken sandwiches, burgers and fries.
Another place you might want to consider is Fort Tryon Park, technically in Manhattan but it's the northern tip of the island and close to the George Washington Bridge. The park itself is beautiful, much of it on a cliff overlooking the Hudson River. It's a great place to walk around, and there's a large playground at the southern end. The park also houses the Cloisters museum, which is part of the Metropolitan Museum of Art. It houses much of the museum's medieval collection. The building is partially constructed of monasteries salvaged from Europe. It is actually pretty kid-friendly--there are lots of outdoor courtyards to wander around. There is a small cafe at the museum that sells sandwiches and fruit. There is also a fancier restaurant in the park itself called the New Leaf Cafe, and there are some random pizza places and small restaurants a few blocks south of the playground. Parking would be easy on a weekday, there are parking lots near the museum and the whole park is easily walkable.
There's also Sesame Place, sort of near Trenton, not sure if this is too far out of your way. I have never been there, but all my friends have and say it's great. There are lots of water activities, so it's best for a warm day, and bring a change of clothing.
(Btw, I just noticed someone recommending Imagine That in Florham Park--I have been there and I would skip it. It's overpriced and nothing special, imho)
Have fun, whatever you choose!
Edited by NicaG - 5/17/11 at 9:30pm