Distance from Boston: 20 minutes
Characteristics: swimming for dogs, off-leash, lots of puppy socialization
Size: 130 acres
Recommended Parking: Google maps will take you to a nice parking lot.
Cat Rock Park in Weston (near Waltham), is a quick drive via the Pike and I-95 from Boston. I've been meaning to come here because I have heard great things about it: swimming, lots of other well-behaved dogs around, and it's legally an off-leash park! Tucker enjoyed having the freedom to romp around with other dogs. It is a great place for early-stage puppy socialization. On a Saturday morning, we saw about 15 dogs, 7 of them puppies, and 4 of them black labs.
The park itself isn't extremely well-marked, except there are a few white trail markers. It's a relatively small park though, and your cellphone GPS will work just fine. I wouldn't worry about getting lost. Some caveats: in early September, it was buggy, more so than the nearby Middlesex Fells and Blue Hills. There is also a lot of poison oak around, so be careful if you need to go off-trail to use the bathroom.
For an off-leash park, it is extremely clean. The regulars do a great job of picking up after their dogs! In terms of swimming, the water isn't great, but it's fine for a pup. If you're looking to swim with your dog, I would recommend driving 20 minutes further to Ashland Reservoir (will post about this soon!).
Walking in and out of the trails in the park, and playing with other dogs took about 1.5 hours.