Random collection of thoughts, there is a pretty active FB group/Discord server that have a bunch of random activities

https://www.facebook.com/groups/3109143809154886

https://discord.gg/TSWJvEcA
Bike-specific groups that host rides in the warmer months

https://www.facebook.com/groups/500325783405970/

https://www.facebook.com/groups/325402935531296/

Meetup is also great for that

https://meetup.com/

Lastly there is a Boston specific subreddit

https://www.reddit.com/r/BostonSocialClub/