Ask HN: Places to see in San Fransisco

Hi!

I'm a competitive programmer who's about to visit San Fransisco for 2-3 days. I was wondering what places I should visit in the Bay Area, I mean, I think it can be a great place to learn and see how startups work, so, any suggestions as to which places I should visit while I'm around would be awesome. I'm looking for a little fun trip with some learning and new experiences maybe.

Maybe being able to visit the startup you work in would be great, I don't know maybe Dropbox, Facebook, Twitter, Box, or any other cool company that allows me take a "tour"...