I will second (or third) the recommendation for warm clothes and a wind breaker especially along the coast near San Fran. I made the mistake of taking a trip to the pier in a T-shirt and shorts. It was hot and sunny about 40minutes inland but when we got to the bay, it was cloudy, windy and VERY cold. Me and the rest of my tourist friends had to warm up at a local grocery store before heading back. If you paln on doing the coastal areas and more inland in a day, layering would be a good idea - warmer for the coast and cooler as you drive eastward.