Here is a small list of retreat centers in and around the San Francisco Bay Area.
For a combined Yoga and meditation retreat, the Sivananda yoga ashram
is well liked
Vipassana Meditation Retreats (Goenka style). It is the only one free of cost.
Solo retreats at Vajrapani Institute can be very nourishing.
Rites of Passage and Vision Quests with School of Lost Borders