Middlesex Fells Reservation, or "the Fells," is one of the most popular dog friendly areas near Boston. It is a quick 5 mile drive up I-93. The Fells itself is huge - boasting over 2,000 acres, you can easily spend 3+ hours here with your pup. On the weekends, it is very very busy. Come around 8AM if you want to avoid the crowds. You can lose yourself in the woods too because there is strong cellphone reception in the reservation, so it's easy to find your way out via Google Maps.
The first main area at the Fells is Sheepfold. It is a 10-acre designated off-leash dog area. The dogs here are generally socialized and well-behaved. People have left water bowls and old balls laying around. Please remember to pick up after your pup and bring poop bags. There are none provided. The best times to come here are on the weekends, although you might see a few dogs weekday mornings (8-10am) and evenings (5-7pm).
My favorite area in the Fells starts at the Long Pond Parking Area. From there, you can take Molly Spring Road to the popular Reservoir Trail. Aptly named, this 5.5 mile trail loops around the 3 reservoirs used as the Winchester Water Supply. This trail is a beautiful, mostly-shaded loop that sometimes crosses with the Skyline Trail and Mountain Biking paths. The entire loop takes around 2 to 2.5 hours. On a typical weekend, you will run into 8+ dogs and their owners also hiking this trail.
There are also many other areas in the reservation, including Long Pond where you can take your pup swimming! Check out the DCR website and explore the area. It is truly a gem!