If you are spending time in San Francisco, June/July are actually chilly months of the year (sorry). The weather always fluctuates, so layers are the way to go. In the City, days will be ~55-70° Once you leave the city, the weather changes pretty dramatically. Outside of the city, the days will likely be ~65-85° and the nights cool down to ~65°. L-A-Y-E-R-S
If you are staying at the DoubleTree, they have a shuttle to/from SFO and OAK airports for ~$30/person. Rohnert Park is a college town, so there are plenty of Uber and Lyft drivers in the area. It is pretty awesome to take a long drive along the coast, so there are also many benefits to renting a car.