Garden Conservancy's Open Days program. Since 1995 this program has organized self-guided tours through gardens that you would never otherwise get a chance to view. There is a $5 admission fee to each garden but you don't have to register or buy tickets in advance.
In the East Bay two private gardens in Lafayette and one in Oakland will be open on May 31. These gardens appear to be a great combination of plants, sculpture, and amazing design concepts. Check out the East Bay Open Day page for information and a couple photos of what's in store.
In Marin County you can visit four gardens in Belvedere, San Rafael, and Tiburon on June 7. In addition to great landscapes, these gardens also offer some sweeping views of the bay and beyond. Check out the Marin County Open Day page for information and photos.
San Francisco's Open Day is June 21 and it offers up five gardens that show just how exciting and creative urban gardening can be. Most of these gardens are really tucked-away secret spots you'd never even know about but they have been expertly designed to make the most of limited space. Check out the San Francisco Open Day page for details on this tour.