As you mentioned there's the playground at the Middle School. Recently I came across a playground that is a few minutes out of Berkeley Heights, not as convenient as the Middle School, but would be great for a weekend, it's only about 8 minutes out of town, maybe less... If you like the outdoors, the atmosphere is very nice, especially this time of year with the leaves changing colors. The playground is the Watchung Reservation Playground, located in the reservation. If you take a look at the following map, you'll see that it's off of 645, close to New Providence Rd and Summit Lane.