Thank you all for your kind suggestions! I'm very excited to jump in - what a great town. I've consolidated the suggestions below, for my own sake and for easier consumption by future readers with similar questions!

Sports and Fitness

• Appalachian Mountain Club organizes activities and events to foster the protection, enjoyment, and understanding of the outdoors
• Boston Ski and Sports is a club that organizes ski getaways, indoor and outdoor intramural sports leagues, adventure travel and social events
• MIT Outing Club (MITOC) helps the MIT/Cambridge community enjoy the great outdoors
• November Project is a free fitness movement to help people stay active though group exercise
• Urban Axes Boston Leagues is an axe throwing community that organizes 8-week league competitions

Games

• Pandemonium is a board game store that hosts tournaments, game nights and book events

Volunteering

• Cambridge Fridge is a mutual aid focused on food sharing, open to volunteers
• Food for Free is a non-profit fighting hunger in the Greater Boston area, open to volunteers

Other/Misc.

• The Democracy Center is a community space that hosts various events
• The Foundry is a hub for creative exploration that hosts various community events