Get out of Mill Valley if you want summer warmth. It's beautiful, magnificent in fact, and I love the smell of the dried redwood bark in the canyons, but the temp is about 10 degrees cooler than, let's say, San Rafael, all summer long. If you like sun, at least get to CM or Lk, Greenbrae or central SR. Otherwise, bring a jacket and enjoy the overcast. In any case it is better than SF. Right?