While it's a bit annoying to get yelled at by a ranger for having my dog off leash about 100 feet from the field where you're allowed to let them roam free, this still remains near the top for me in terms of the best places on the North Shore to take your dog friend.
The location and entrance is super easy to find, it's right off of Rt 28 between Stoneham and Medford (across from the 93S exit ramp). The problem is, parking in either of the two main lots can be packed depending on the time and day of the week you go. Early in the morning? Usually not a problem, get there before 10 AM on the weekends. After work, right before sunset? Also usually a good time to go? Avoid at all costs "peak times" during the weekend though, you'll end up not finding anything, getting frustrated and making up your own spot only to come back to a parking ticket. At least when we had a dog meetup here once, the ranger came over to tell us they would be ticketing cars parked illegally and they had to be moved instead of just doling them out.
Did you arrive at a weird time and no other dogs are around? Head into the woods behind the large field and take a trail walk. Again, watch out for rangers who may yell at you for not having the dog on leash. There are about 5-6 different entrance points into the Fells trails. There are public water sources surrounding it so you are a little limited to where you can go but there is a nice hike to Bear Hill Tower from here which will take you about 15 minutes.
The weekends can have anywhere from 5-50 dogs at a time and there seems to be a "pack mentality" that brews once the number of dogs reaches 20 or more. So be prepared for that, if your dog can be aggressive or timid, I wouldn't recommend visiting at a peak time. The good thing is, you can keep your dog away from many of the others if you decide to just throw a ball, it's easily 1/4 mile long from end to end, if not more than that.
Bring doggie bags and for crying out loud, take it with you. I don't know why people think it's ok to leave bags of dog feces all around for someone to pick up after them, what's so hard in taking it out when you leave? There are also no places to get water for your pal so your best bet is to bring a big communal bowl or bring lots of water and someone has probably left or has a bowl there already.
Put aside a half hour for your dog to go crazy with others or 3-4 hours if you want to hit the trails with them and combine it with some play. All around, it's the perfect place for the both of you to burn off some extra calories. *Woof, slurp, grunt, howl, pant* read more