San Francisco is a city full of museums, large and small, catering to pretty much every age and interest imaginable. While a few are always free - for example the Cable Car Museum, the Chinese Cultural Center Gallery and San Francisco Fire Department Museum - others have certain days or times when they waive or reduce their usual entry fees.
Tuesdays are a particularly popular day, with the first Tuesday of each month free for the Cartoon Art Museum, the de Young, Legion of Honor and Museum of Modern Art. The first Wednesday of the month can find free admission at the Exploratorium and Bay Area Discovery Museum.
For those who prefer the outdoors, there are free guided walks offered by the San Francisco Public Library and the San Francisco Botanical Gardens as well as an enormous variety of nature walks offered by county and state parks.
Outside of the city, both Stanford and Berkeley have free museums on their campuses, and the Museum of California in Oakland is free the second Sunday of each month.
This is by no means an exhaustive list, and programs are no doubt subject to change, so please double check before you head out to sample and enjoy!