We are having a week in SF (after NY and Vegas) towards the end of July and want to hire a car for a couple of days. We want to take in Muir Woods and drive up to Napa and then the next day head south to Monterey etc. - nothing too extensive just to give our girls (16 and 12) a flavour of the place without boring them with too much 'culture'. However, we were wondering whether to hire the car over the weekend or mid-week. Any opinions?

I'd do mid-week to avoid the traffic.

Ditto mid week for traffic and crowds; don't know about cost if that was the point of your question.

We just returned from SF/Napa/Ft. Bragg. I researched this and it seems to be cheaper outside SF during the week and in SF on the weekends. We rented a car from the Hyatt in Fisherman's Wharf and returned it at the airport. $135 for a premium car (Bonneville) from Budget, unlimited mileage.
I would definitely sugget Matanzas Creek Winery in Santa Rosa if you are in the area. They have beautiful lavender gardens in bloom, in addition to their yummy wines.
Have fun!

Sorry, I want to add that the car rental was for a Tuesday through Sunday period.

And one more thing. . . if you have a car anyway I would suggest taking (at least part of) the 49-mile scenic drive in SF to see ALL of what the city has to offer!

Thanks Tim and Liz. You also had some good pointers for our trip over to Alcatraz. You seem to have had a great trip, hope ours goes as well.