Can we wait until the weather is not..gross and cold? LOL
As for in the city of Boston..
The Common is the biggest, in the middle of the city and it's beautiful any time of year but dogs need decent voice control. Dogs are allowed off leash in specific sections but there is no fence around them, they are in the center of the park and it's very safe but dogs needs to be under voice control because people are out and about. But it's a huge park, lots to do, eat, see, swim etc.. There are so very pretty public gardens around and lots of shopping and eating on Newbury street! It's a great spot!
Otherwise, Peter's park is the best off-leash traditional gated dog park IMO
http://www.yelp.com/biz/peters-park-dog-run-boston
It's nothing spectacular really lol big gated area, water fountain, little dog section, it's kept very clean
Dogs are allowed on all public transport so it's easy to move about!