[dougmccune]

Someone else mentioned Maker Faire, and I second that. Even if you don't think that kind of thing is "your thing", there's so much there that's genuinely interesting to any inquisitive person that you're bound to find it fun. The only drawback IMO was that it's massive and crazy crowded, so if you're not a fan of large crowds it can get overwhelming.

Also, change your ticket and fly back on Monday so you can experience Bay to Breakers. It's one of the best/most ridiculous events our city has to offer.