This playground right off the Charles, across from the Arthur Fielder footbridge, is specially designed for older kids. It is super-cool, with climbing equipment (web, huge slide, rope balance beam, hammock) that had lots of adults playing too, without even putting on the "I'm just helping my kids" act. There is a really high, long zip-line, cool twirly things (that’s a technical term), and various other entertainments. If you bring a toddler along, though, he might have very little to do (my experience for the first 10 minutes) or he might decide to climb straight up the climbing web designed for older kids and give you a heart attack (my experience for the rest of the time there).
Here, I also have to give a shout-out to another Boston-area-playgrounds blogger, whom I discovered while searching for this playground! Check her out: http://playground-hoppingmass.blogspot.com/2012/07/esplanade-playspace-boston.html . She seems to range farther than I do, and has some entries I plan to check out soon.