Run clubs
San Diego's run club culture is enormous and it's the highest-return social move available to a new arrival. Most are free, weekly, welcome all paces, meet at or finish at a brewery, and are explicitly social rather than competitive. Nearly every central neighborhood and most beach towns have one, and many running and outdoor stores host their own.
Local tip: Go to the same club three weeks running before deciding it didn't work. Week one you talk to nobody, week three you're getting invited to dinner.
Adult rec leagues
Beach volleyball, softball, kickball, soccer, flag football, pickleball and more run year-round through several San Diego social sports organizations. Crucially, you can register as a free agent and be placed on a team — the entire structure is built for people who don't know anyone.
Beach volleyball
The most San Diego version of the above. Open play at Mission Beach, Pacific Beach, Ocean Beach and Coronado runs constantly, and open-play courts are genuinely welcoming to people who ask to rotate in.
Climbing gyms
Bouldering is structurally social — you rest between problems, you watch other people, and the standard opening line is asking someone about a route. San Diego's gyms have strong communities and most run intro classes and social nights.
The slowest of these to pay off and the most durable. Surfing a consistent break at a consistent time means you see the same people constantly, and lineups turn into friend groups over months.
Volunteering
Beach cleanups, food banks, animal rescue, trail maintenance, habitat restoration. Recurring shifts with a shared purpose produce friendships faster than most social settings, and the age range is wider than anything bar-based.
Classes and hobbies
Language exchanges, cooking classes, improv, pottery, dance, sailing lessons. Anything with a multi-week format has repetition built in, which is the whole mechanism.
Group fitness and yoga studios
Studios with a class-pack model and the same 6am regulars build tight communities. Beach and park yoga in the warm months is even easier to walk into.
Meetup and interest groups
Hiking groups, board game nights, book clubs, photography walks, tech and industry meetups. Quality varies wildly — the ones with a consistent weekly or monthly cadence and a core of regulars are the ones worth your time.
Becoming a regular somewhere
Pick one brewery, coffee shop or neighborhood bar and go the same night every week. Trivia nights are the fastest version of this since they force team formation. It's slower than a run club but it works.