Looking for treasure!

I like to do fun things.  Luckily, I live in one of the funnest areas in the States -- the San Francisco Bay Area.  Spurred on by a friend's request, I'm compiling a list of fun things to do around here.  Since there are so many things one can do, I'm using the following selection criteria:

• Fun: The most obvious and subjective criterion.  Since I'm compiling the list, it will be biased towards things I find fun -- active, athletic, quirky, adventurous, or helpful preparing for the apocalypse.
• Drop-In: Things that you can do once and get something out of it. Some, like flying trapeze, you certainly won't have mastered and you might be inspired to pursue more seriously, but you'll still come away having a great time.  This does mean, however, there are a lot of great clubs that I won't be showcasing, because to get something out of them requires a bit more commitment.
• Recurring: Events that happen regularly, and ideally frequently. Some, like the SF Chinese New Year's Treasure Hunt, happen once a year, while some, like Handstand classes at Athletic Playground, happen multiple times per week.  One-time events, even awesome ones, won't be featured.

I'm also always on the lookout for new and fun things, so feel free to give suggestions!