Ask HN: 1 day in SF, what do see on the tech scene?

If you had one day in San Francisco and Silicon Valley, what would you do, visit, or experience?

More: I will be arriving in San Francisco for a weekend conference in early June, and will have a free Friday to play and explore. I've seen several fantastic landmarks and tourist spots on previous trips, but would love to check out a bit more what SV is like and the companies that create the pulse of the area.

Thoughts and suggestions?