Stone Zoo is a wonderful idea for a half-day trip just outside Boston. The zoo is small, but it houses a nice collection of interesting animals, and the enclosures are designed so you can get a really good view of the animals - especially the large cats. You cannot compare this zoo to some of the world's best zoos (i.e. San Diego, Toronto), but if you want a nice day trip, something to do outside of Boston, I highly recommend the trip.
One of my favorite parts of the zoo is their petting zoo area (yes, I am a child at heart!), where you can get up close and personal with some popular farm animals. The animals are quite friendly and appreciate the interaction (some of them will even wait at the fence, hoping for some petting from the passers-by).
My one gripe with the zoo is its placement right next to the highway. The wolves' exhibit, for example, is located right next to the highway, so that exposure to constant noise can't be healthy for the animals - but then again, I am not an animal expert, so perhaps I am wrong...
Check out my attached photos for some shots of the animals!