-You might get some interesting and fun things in here: California Academy of Sciences and The Exploratorium. Every Thursdays they have special events.
-This is another website where you can find something: http://www.sfweekly.com/
If you want to socialize www.meetup.com (already told by Jemaclus) is great. You can also organize or find meetups to programme instead of programme alone for example. There are meetups for whatever think you want to think, so just try some of them until you find something and some people you like!
With that said, comprehensive list. Even though I can't use it, I think it highlights some great activities SFBA HN readers would enjoy.
* Run along Lake Michigan
* Check out the super awesome theatre scene in the Loop
* Check out awesome off-Loop theatre companies, too!
* Nerd Nite Chicago! http://chicago.nerdnite.com
* ComedySportz Improv contests! http://comedysportzchicago.com/
* See a Cubs game at Wrigley Field
* Volunteer at One Brick Chicago: http://chicago.onebrick.org
There's a bunch, you just have to go out and find it. :) The problem I had with Chicago (I used to live in Uptown) was that it snows for 9 months outta the year, and you're too cold to actually go out and do anything. And once you get home and get dry/warm, you don't wanna go back out again. That suuuuucks.
The trick was to stop and do these things on the way home from work, so you only go home once! ;)
* Write. Every weekday morning, I go to a coffee shop downtown and write for 30-60 minutes. I've written two full-length novels and two novellas so far, though they are all as-of-yet unpublished. Someday...
* Run. I run 3-4 nights per week, either along Embarcadero or through Golden Gate Park. I started off with the Couch to 5K program in January 2012 and I've worked up to four half-marathons in the last 12 months.
* Disc golf. There's a disc golf course in Golden Gate Park. Grab a beginner set (20 bucks from Amazon: http://www.amazon.com/Innova-Disc-Golf-3-Disc-Colors/dp/B000...), grab a friend, and go play. It's super fun.
* Nerd Nite. http://sf.nerdnite.com - 8 bucks. It's like the Discovery Channel -- with beer! (Side note: it's also where I met my girlfriend, and I go every month.)
* Rock climbing - check out Planet Granite (http://www.planetgranite.com/) or Mission Cliffs (http://www.touchstoneclimbing.com/mission-cliffs)
* Meetups - I've attended a dozen Meetups, including the SF Rust meetup. Very fun way to meet new people.
* Biking - Go to Roaring Mouse bikes in Crissy Field (http://www.roaringmousecycles.com/) and buy a bike. Then bike to work, bike to Golden Gate Park, bike across the GG bridge to Sausalito, to Tiburon, to Marin. There are a million awesome places to bike in the Bay Area, and there are huge communities around it. It's something you can do by yourself or with a group. Super fun.
* Anything from sf.funcheap.com. Like the site says, it's a list of fun, cheap things to do in the Bay Area.
* Take online classes (http://coursera.org). I'm currently taking the Machine Learning course and the Compilers course. It's very fun to learn new things!
* Become a Stormtroooper (http://www.goldengategarrison.com) - I'm not a member, but I know a lot of people who build their own Stormtrooper armor and then do volunteer activities with the 501st (the official Stormtrooper organization). Very cool way to give back to the community, especially if you're a Star Wars geek!
* Volunteer! There's a great organization in the city called One Brick (http://sfbay.onebrick.org/). You basically pick an event and just show up. It's volunteering a la carte! You're not making long term commitments -- just one event at a time. And you meet TONS of good, kind-hearted people. It's a win/win!
* And being Hacker News, I would be remiss if I didn't mention: start your own business. Find a problem domain you know something about, and solve that problem. Get out there, meet some co-founders, crank out some code, save the world, and get the cheerleader. Or maybe it was save the cheerleader? Whatever.
* And so much more. I do all of the above things regularly (sometimes daily, sometimes weekly, sometimes monthly), but there are a huge list of things I just don't have the time or memory to mention. There's a whole world out there. Get outside and explore!
Good luck!