As a hiker at heart living in a city, I am always looking for close escapes that will give me a good city release and wilderness dose. If you are looking for a nice selection of trails and gorgeous coastal hiking just outside of San Francisco, I highly recommend dropping by Tennessee Valley. The rolling hills are lovely and the coast is just waiting to be explored, beaches often deserted or hosting a few people scattered here or there. It's a lovely loop that allows for quiet contemplation and a solid sweat (if you want to pick up the pace). Also plenty of options for mountain biking.
I highly recommend swinging through Mill Valley for a pint at the brewery afterwards (Mill Valley Beerworks is a sure bet).