Personally:
-sports: climbing gyms/trips/facebook groups/etc, bike groups (not exclusively biking alone), hiking trips, etc
Anecdotally:
-drama clubs/local theatre productions
-run clubs: while running is typically fairly isolated, there are social run clubs in cities that often go for a drink (no booze necessary) post-run
-book clubs: random collections of people that discuss a book together
-dinner clubs: sharing food with folks in a way where different people cook for others in turn
-partying: most places have a community of folks that enjoy dancing and/or recreationals
Note that I believe that the strong relationships can come from overcoming a shared struggle, so if you can think of something that's difficult and with one or more other person/people, you will probably form a community around it over time (co-founders and cohorts are a great example).
Anecdotally: -drama clubs/local theatre productions -run clubs: while running is typically fairly isolated, there are social run clubs in cities that often go for a drink (no booze necessary) post-run -book clubs: random collections of people that discuss a book together -dinner clubs: sharing food with folks in a way where different people cook for others in turn -partying: most places have a community of folks that enjoy dancing and/or recreationals
Note that I believe that the strong relationships can come from overcoming a shared struggle, so if you can think of something that's difficult and with one or more other person/people, you will probably form a community around it over time (co-founders and cohorts are a great example).