There's homeschoolers in Cambridge and active groups around west of there - in Arlington, MA for instance. There's a homeschool book club that meets at Cambridge Public Library for upper elementary/tweens. There's a bunch of homeschoolers who meet at a playground in Cambridge - I think Homeschooling Together (
http://www.homeschoolingtogether.org/) has the details.
There's also group that meets Wed at Beaver Brook in Belmont near border with Waltham when the weather is good; during the winter they meet at Robbins Library in Arlington, MA on Wed. I think they'll be meeting at Walden Pond in Concord on Fridays soon since the weather seems to be improving.
I'd suggest looking at AHEM (
http://www.ahem.info/index.html) and MHLA for homeschool groups (
http://www.mhla.org/support/supportgroups.htm) and other details on homeschooling. This is my third year un/homeschooling my ds; I basically cut/paste the ed plan and progress report from AHEM.
MIT, Harvard, Museum of Science, various public libraries, and other institutions have many events and other activities - usually throughout the school year - for plenty of learning opportunities.
