There's lots of great suggestions here, but I'll add some thoughts about low-key options that don't involve drinking:

• School of Honk (Sundays, Somerville): Music-focused meetup with loaner instruments and friendly instructors - beginner-friendly!
• Park Run (Saturday/Sunday, Boston/Cambridge): Casual, timed 5k Walk/Run which often ends with a group going out for brunch and is attended by locals and visitors alike.
• Knight Moves Board Game Cafe (Daily, Brookline): Drop in and play games with friends or strangers. I haven't tried this one myself but hear good things.

I have some other ideas but DM me so I can get an idea of what your interests are. Good luck!