San Francisco and "the valley" are very different and far enough apart that you don't want to make the commute between them any more often than necessary. Plan on multiple consecutive days in one before you visit the other.
It's nearly pointless (and a huge hassle) to have a car in SF, and it's useful if not essential to have a car in the valley. Zipcars are cute but they end up being far more expensive than regular rental cars if you need them for more than a few hours. They also require a significant amount of up-front planning, as you have to obtain additional hardware before being able to drive one away.
Best place to meet and network are meetup.com, and other entrepreneurs' pitch sessions. Hacker Dojo and a few other co-working places are pretty good too.