Over in Westville is a lovely little park. It's got trails for wandering (or mountain biking), picnic tables in the sun or shade, some streams and ponds, and enough open fields for several independent games of soccer, wiffleball, rugby, or what have you. Near Ella T. Grasso, there's a playground with your typical swings and junglegyms, and the coolest fountain/concrete sculpture I've ever seen, meant for kids to splash in on hot summer days. On the other end of the park, near the Whalley/West Rock intersection, there are basketball courts with a nightly game, and a skateboard park (though, if you're a voyeur, I haven't actually seen the kids do much that's either impressive or life threatening).
If you're looking for a change of scenery for running your dog/child, come check it out. Otherwise, it's not worth a special trip if you're not nearby.